I liked the idea a LOT :D But the actualy gameplay for one player was just way too much for me, being able to coordinate moving in both (especially when I got to the level where there was spikes), while trying to avoid arrows and killing the enemies was overwhelming :D
It would for sure be a fun sofa co-op game with some trolling (just drink all the health potions on your side to spawn minibosses for your mate :D
Also just a heads up for the controls, some countries have different places for keys (atleast the right side of it), I'm rocking a Finnish keyboard, so for me I found them by testing :D, because I think they were like P L Ö Ä letters.
Would need some rethinking how to get it working with just one player, maybe somehow the other side would have to do something to help end the spawning of the enemies on the other side (like a small puzzle) and the other side would focus on killing. That way you wouldn't have to worry about the other side dying, but if you don't do the puzzle or whatever it would be the spawning never ends on the other side and you'll be fighting forever :D