There's an item in Races of Stone that could help: the gnomish quickrazor. It's held on your wrist, and sheathes itself after each attack you make with it, so your left with your hand free again.
I can hold two swords in one hand if I just want to carry them. Why not allow him to shift one weapon to his other hand, cast the spell, and then retrieve the weapon.
I'd call that two move actions, both provoking AoO, in addition to a standard action. Taking two swords in one hand seems difficult and not conducive to defending yourself.
