Or, because they are still making the game, they could simply change two handed weapon attacks to a 2ap action. Made for dealing large swaths of damage with high criticals.
Yet that would step on the heels of special moves to an extent I guess...
As for their difference simple.
One hand with Shield = Full defense
Dual Wielding = Balance
Two handed = Full Offense
So lets just assume they stick to 1ap
Think about it. "All" actions take one AP now. So dual wielding does as well, meaning putting points into it is useless for diminishing AP penalties, except as a sort of hybrid of the system.
As for making One Hand with a Shield worth it. Expand what it can block. Give it a chance to mitigate some magical damage and attacks.
Suddenly the Shield becomes extremely useful.
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I don't actually dislike the reduced AP system. It just means it has a different idea of an action economy AND it makes true hybrid classes actually viable.
Sure you COULD make real hybrids in Divinity 1, but because you were split two ways the best method is really just to have a main skill and a minor one. Going 50/50 made you extremely subpar.
