I once had cat who got really jealous if he caught you stroking the other cat, his breathing would get heavy and he'd bite the other cat and then try to bite your hand if you came near him for about half an hour afterwards (he never showed any agression, not even scratching normally). I believe it was jealousy and never really found a solution I'm afraid. Best thing I can suggest is that each of you slowly try to give the opposite cat a little more attention to get them used to it. You could try rewarding them with a treat for good behaviour and distract them from attacking the other cat.