I don't know if this is productive or not, but I thought it would be a good change of pace to ask: What's the *worst* character class out there? Follow-up: Ok, it *looks* like a bad combination. But do you think you could make it work?
I kind of think that it almost doesn't matter, since if your classes don't combine well, you can always pump one a lot and then just pick up those one or two skills from the other tree that actually help you out. For example, a melee Champion character could take just a few points of Heart Of Oak, Regrowth, and Accelerated Growth. At high levels, when the Warfare tree looks solid, he could come back for Heart Of Stone and Refresh. Plague with Fatigue might make an ok crowd control debuff at early-mid levels, and then susceptibility could probably help a lot at high levels... See, there's more than it seems at first.
It seems like every time a combination seems like it wouldn't work, it turns out better than I thought if I just think about it harder. But TQ is just kind of fun like that.
One thing I haven't thought about yet, though, is mixing up magic types - seems kind of like you could spread yourself too thin in ways that don't stack, like storm/earth. The only thing I can think of at the moment is to put all points in Earth for a long time, and then eventually get Eye Of The Storm maxed out. Maybe get a couple defensive enhancements along the way, too.
The trouble with magic/magic is that it's not as obvious what to do as when you have a hybrid fighter type. But it might still work.