Hmm, interesting problem. If I were in your situation, here is what I would consider 'fair':
1) You cannot claim a card unless it's actually in your spellbook. If it's not actually included in your spellbook, it's fair game for the other player. Period.
2) The mage trained in the relevant schools always gets first pick of all spells in their trained school. If the other player wants some from that school, he has to wait and possibly do without. The exception is the Wizard: certain key spells (Teleport, Dispel, Dissolve, maybe Nullify) need to be shared evenly. In the case of odd numbers though (three Teleports), the extra card in the Arcane school would go to the Wizard if he wants it. Also, Warlock trumps Wizard for Fire spells. (Recommendation: DO NOT BUY CONQUEST OF KUMANJARO!!! ; )
3) Once those hurdles are passed, any remaining contested cards are put together. Flip a coin to see who goes first. The winner chooses one card from the pile, then the loser chooses one, then the winner, then the loser, and so on until all the cards are chosen. Every once in a while (say, a week or a month, depending on how often you play) you agree to possibly redesigning your spellbooks and doing this whole process again: priorities change as you play for a while, and it gives a player another chance to try a card they really want.
