Yes, it's really annoying to run a party with two lone wolf mages. Not just for that reason, but a lot of enemies are immune to (or healed by) everything except physical/void damage too.
Luckily you can get around it by summoning minions that do physical damage (armored skeleton that witch can summon seems to be the best), but if you don't have such a spell or physical damage-dealers later in the game then you literally can't progress.
