Um okay.. but those are opinions...
Yes, they are. So?
The OP didn't ask for ANY build at all; s/he just merely asked about an issue with spells. And yes, you can play whatever you want, but that doesn't mean it's going to be a good idea mechanically.
Clerics and wizards are casters. Neither duelist nor DC advance spellcasting, and that's a serious drawback, whereas the possible benefits aren't that spectacular.
Duelist gives -some- AC -if- unarmoured and unshielded. Armoured wizards aside, a few duelist levels may have some use in epic, but other than that, more and stronger spells is always a better option. Besides, if you really need duelist to survive, you're doing something wrong in the first place. Yes, technically, duelist and wizard have some synergy, but that doesn't mean it's a good idea to combine them. Not if you have to sacrifice more important things, at least.
Clerics don't need charisma at all, but either way, you mention Turn Undead, Lay on Hands and Smite. DC doesn't advance TU (which is not that great anyway, but still), clerics can smite with the right domains (not that smiting is that great either), and about Lay on Hands... well, that's a pretty laughable ability when you can have the clerical spell list at your disposal. You can absolutely play a cleric/DC, but it's not something I'd suggest, and frankly, I'm not seeing a lot of synergy in practice in that example.
I know what you mean: You're suggesting to look for synergy and mentioned two examples. Fine, you've made your point, but I don't see anything wrong with me or Luminus suggesting not to use those particular examples.