I know I'm among the minority here, but I've never done a cross-class combo, at least not intentionally. I think my companions have done them before, as I received the achievement for it, but I've never set up their tactics for them. I think it was all luck.
I'd like to give cross-class combos a try on my next playthrough, but I have no idea how they work. What do they do? How do I set them up? I'd really appreciate some advice. 
You prime a cross-class combo by inflicting a certain status effect on an enemy or group of enemies(indicated by an icon on top of the enemies head), then complete the combo by using an appropriate attack to inflict massive damage. The name "Cross-Class Combo" comes from the fact that you need to prime a combo with 1 class and complete it with another due to that class having exclusive access to the status condition.
* Only Mages can cause BRITTLE
* Only Warriors can cause STAGGER
* Only Rogues can cause DISORIENT
It's fairly simple, really.
Some abilities, have upgrades that say something like:
Damage 300% against Brittle targets
For example, an upgrade to Mighty Blow.
There are spells, like upgraded Winter's Grasp and Cone of Cold, or upgraded Petrify, that will make an enemy Birttle - it's indicated by an icon floating above enemies head.
When you use the ability that Mighty Blow on that enemy, it will deal far greater damage.
Other "statuses" are Disorient - applied by Rogues, and Stagger - applied by Warriors.
Eploiting Cross-Class Combos can lead to devastating damage. For example Varric's Archer's Lance can easily deal over 10 000 damage against Brittle target.
I think the easiest one and the one I use to use is upgraded shield bash with aveline and put Chain Reaction in Tactics.
Ain't got much to add, because the basics have been explained, but might as well share, that my most frequent cross-class combo is really simple to do: requires a mage, and a warrior. Mage uses an ice spell that can leave opponents BRITTLE, and if it connects and the "brittle" icon appears, warrior uses a power attack like Mighty Blow, and if it hits, the enemy gets shattered
If you want, tell us what class you are going to play and what companions you like having around the most so we can give you some more specific examples.
My 3 favorites are
As mage
Cast wintergrasp (brittle) -> have varric use archer's lance
As rogue
Have merrill cast hex of vulnerability then upgraded petrify (brittle) -> finish with assassinate
As warrior
Have varric cast fatiguing fog (disorient) -> finish with scatter (can hit multiple targets)
You supply a cause, and then an exploit. Two ways to do are 1) program your allies to set causes up frequently, and then exploit them yourself; or 2) program your allies to exploit when an enemy has a cause on them "ENEMY: Status Disoriented" for example).
I'll give you some optimization tips in case you want to try them out.
The most efficient way to use cross class as mage is getting the talents Winter Grasp (with elemental mastery), Hemorrhage (upgraded), Arcane Bolt (upgraded), and Walking Bomb (virulent upgrade only, don't take corrosive). Also going Force mage specialization get Gravitic Ring and Pull of the Abyss
Your warrior will need the talent Cleave (upgraded) and Shield Bash (if you use Aveline), Your rogue will need fatiguing fog (upgraded) and Archer's Lance (upgraded)
For Rogue tactics
Enemy: Clustered with 3 or more -> Fatiguing Fog
Enemy: Brittle -> Archer's Lance
For Warrior tactics
Enemy: At close range -> Cleave
Enemy: Clustered with 3 or more -> Shield Bash
The playstyle is that when you see enemies coming at you, you cast a gravitic ring and slow them down, then pull of the abyss and pull them all one on top of the other.
Your warrior will use cleave and start attacking them. Since his attacks are aoe, he can cause stagger to a lot of them. Your rogue will cast fatiguing fog and make them all disoriented.
You will pick the strongest target, cast walking bomb, then wintergrasp (he will get Brittle so your archer will cast a VERY high damage archer's lance), and arcane bolt. This should be enough to kill the target. The Walking Bomb will explode and deal huge aoe (it can outright kill everyone depending on the circumstances). Finish them off with a Hemorrhage if Walking Bomb didn't kill someone.
In case you don't like to play with warriors on your team (like I do), you will notice fast that they are not that important for synergy. They can be substituted for another mage with Firestorm (upgraded, with pyromancer and elemental mastery), and when you use pull of the abyss, cast 2 firestorms on the enemies (yours and the other mage's). This has the potential to outdamage a Cross Class Hemorrhage. Having a rogue with you on the other hand is pretty important. Either Varric or Sebastian will do, I prefer Varric.
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For your rogue playthrough, the most important cross class you can do (and the only one that really matters), is Brittle into Assassinate.
Merrill is the best enabler cause she has access to both Hex of Vulnerability and Petrify (upgraded) while Anders has access only to petrify.
Getting Arcane Bolt (upgraded) and Stone's Fist (upgraded) for your Mages is good too cause you are going to cause a lot of disorients and they can capitalize on it.
The tactics should be like this
Enemy: Rank Elite or higher -> Petrify
Enemy: Brittle -> Hex of Vulnerability (only for Merrill)
Enemy: Disoriented -> Arcane Bolt
Enemy: Disoriented -> Stone Fist
To which you can add Marked for Death if you like. Then use an assassinate. This will do around 15000 damage without gear optimization and other stuff. A very optimized character can hit north of 50000 damage with this.
Everything else depends on your specialization. If you go shadow, you want another rogue with you with the talents Fatiguing fog (upgraded) and Chameleon's Breath. This is because shadow does more critical damage when he is obscured.
The tactics should look like that. Notice that you will have to add these lines last.
----other tactic
----other tactic
Hawke: Obscured -> Skip Tactics
Hawke: Any -> Fatiguing Fog
Hawke: Any -> Chameleon's Breath
This will make sure that Hawke will be obscured all the time, and there won't be any overlapping to waste the talents. Also notice that if you go Shadow, with so many obscuring effects and decoy you can actually become the main tank. I've been doing this on Nightmare, it should be even easier for lower difficulties.
If you go for Duelist you will need a warrior with a surefire way to cause stagger. Shield Bash (upgraded) is the best. Pommel Strike (upgraded) will have problems against bosses. And Cleave (upgraded) is always an option but it may lag a bit due to the randomness of the talent.
For tactics
Enemy: At close range -> Cleave
Enemy: Rank is Elite or Higher -> Shield Bash (or Pommel Strike)
While you are learning to use the cross class, the most difficult thing to get down is the timing. Sometimes tactics lag, or the tactic AI picks the wrong target. These can happen and as you get used to them you will be fast enough to make adjustments on the fly. Pause a lot and get used to the visual effects of the cross class abilities and various debuffs.
I don't think I have much to add to the advice above other than to maybe reinforce some of what has been said.
The important thing is to decide which CCCs will be your mainstay and build your team around it. Worry about the CCCs between you and your companions first, then later on when you've got them working you can add inter-companion CCCs (e.g. varric disorient, aveline scatter). Just to make it a little easier until you get used to what works with what.
Remember though that many CCCs aren't all that useful in act 1. For instance, achieving a reliable rate of brittle from winters grasp and cone of cold requires you to have the tree upgrade. Until then, the brittles are unreliable.
An easy and reliable early game one is upgraded shield bash + upgraded chain lightning.
Late game, everyone in my party is working on CCCs. Varric for instance, doesn't use bursting arrow or lance unless something is brittle. Fenris doesn't use mighty blow unless an elite or higher is brittle. Aveline doesn't use scatter or assault unless Varric has disoriented them with confusion or fatiguing fog.
Your best friend as a blood mage is Fenris, ironically. Properly spec-ed for staggers and whirlwind, you'll never need to use haemorrhage other than on staggered opponents. If you don't like using Fenris, then focus on brittles and make good use of Varric.
If everyone is using CCCs through tactics, the fights won't last long.
Not CCC I know but for the love of Andraste, get rid of Aveline's stupid nerf: her shield defense tactic. I always have it as Self:Health >75% Deactivate Shield Defense followed by Self:Health <50% Activate Shield Defense. You'll be amazed how much more effective she is like that.
You'll also probably find you want to turn the difficulty up a notch because CCCs make a huge difference.
Think about crits as well maybe. My end game crit chance with a mage is usually at least 50%. That works well with CCCs. (achieved through upgraded heroic aura, Malcolm's Honor and jewellery (e.g. bloodstained girdle, four fingered eddies talisman, etc).
Boss fights: what I usually do is the following sequence with upgraded talents,
Varric - pinning shot - DISORIENT
Aveline - Assault or scatter - CCC
Mage Hawke - Horror (keep them at bay)
Aveline - Shield bash - STAGGER
Mage Hawke - Chain light or haemorrhage or fist of maker CCC
Merril or Hawke - Petrify - BRITTLE
Varric - Archer's lance - CCC
They're usually dead by that point. If not, just go round again.
As for playing a rogue, I've not really got much to add to what other people have said above.
Thanks again for all of the tips, guys (especially Gtdef and tirnoney for those detailed explanations). I will probably start playing DA2 either later this week or next week (have to finish up all the DLCs for DAO first), and I'm excited to experiment with the cross-class combos.
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