I'm working on an action-adventure game, and I was considering adding a co-op option. What I'm trying to decide is if when another player joins the game, if the game should get harder. On the one hand, I don't want the game to just get easier. On the other hand, I don't want to break continuity by making enemies suddenly stronger, eg I was able to take this down it two hits before, why does it take four all of the sudden.

I do not want there to be an "AI co-op" that comes with you in single player, I think that would make the game worse since it would detract from player agency.

How can I handle this? How can I keep the game challenging when a second player joins in?