As many of the wizards use the same card decks it is important to deal spells in a strict order. Deal to the highest level wizard first, then the next highest level, and so on. This may result in some wizards having fewer spells than they otherwise would or even none at all.
If the armies include two or more wizards of the same level, then roll a D6 or use some other suitable method to decide which wizard is dealt his cards first.
If you have two wizards of different Colleges then you'll have no difficulty keeping their cards separate. However, if your army has wizards of the same College you'll have to be careful to keep their spells separate. This is important - a wizard cannot use another wizard's spells even if they are from the same College.