Well, one thin g I can answer ...Whenever you know a spell that is in two books, whichever (asssociated) skill you are best at, you use for that spell. So, say you learn Set Portal and your Wizardry skill is higher than your Alchemy skill. Every time you cast this spell, it is done using your Wizardry skill (so that is the one that gets the practice).
As for the combos, there are only 6 combinations:
Mage + Priest, Mage + Psionic, Mage + Alchemist, Priest + Psionic, Priest + Alchemist, Psionic + Alchemist
And if you are having them concentrate on only two different ones each, then the combos are fewer:
1) Mage + Priest, Psionic + Alchemist
2) Mage + Alchemist, Psionic + Priest
3) Mage + Psionic, Priest + Alchemist
I think 1) or 2) are your best bets, as they cover more ground. 3) seems redundant, in that I see the Psionic and Mage as mainly offensive magic and the Priest and Alchemist more as defensive ( though the Alchemist does pack a good punch as well).
Tools for Wizardry(r) 7