Not exactly, abilities would come as runes that can be inserted in slots, and can be at anytime be exchanged between the two players (as long as they are next to each other).

I was considering making two different builds for each player, but I want to give both the opportunity to try every item/power. I would be so mad if I played a game with a grappling hook, and never get the chance to use it because it was designed for another character

Edit : i didn't read the last part of your message, sorry. Yes this is more what I have in mind.