Hmm.
I think the disable state might be what you want to use here. When it's on a battler the split attack (and the extra hit) don't function at all, so the weapon unequipping shouldn't happen either. Since you're using specific attack skills to attack with the main and offhand weapon, using a.MHCalc() and a.OHCalc() in place of a.atk in the formula for those skills should split damage correctly as well. You might need to do some weird workaround in your action sequence to get the offhand weapon to show up correctly during the action sequence for that skill though.