Being able to cast 2 powers without cooldowns would be immeasurably broken. Biotic and tech explosions deal so much more damage than guns that every class not dual-casting would have gimped damage output in comparison.
Have you even played Mass Effect 3?
No they don't. They are great to start with because you level up more quickly than you get weapons, and they solely depend on your powers. But once you stock up with high level guns, ammo, mods, etc dps from guns overtake combos by quite a lot.
Note that you can already "cast" 2 powers without cooldowns by using grenades (and some special powers). And yes, you could consider some grenades broken, but no more so than some weapons, and not because of the biotic explosion.
I don't think there is any point in making any combination of powers work without a cooldown. There can be some cases where it is quite cool. The Awakened Collector is awesome the way it is able to muck around with cooldowns on dark sphere and seeker swarm. There are combos like Throw which instantly refreshes the GCD. There are powers that don't use a cooldown but some other resource like Nova or Phase disrupter. But there are certainly some powers that should never be allowed to be chained together.