Me and my friend are playing together and we find that our builds together just destroy everything we touch and it isn’t very satisfying. Is there a way to increase enemy health pools through files or mods?
You’re going to get increased health through ascendant mode
Welcome to the forum. 
What difficulty are you playing on atm?
Game isn’t balanced around multiplayer regardless of difficulty. You have to use mods.
Give it a try in SR42+, that should be some challenge😅
Veteran.
I appreciate the other difficulties will increase these numbers but it’s a bit boring rn
I’m afraid I don’t know what this means 
What mods would do this?
Ahh it’s an endless dungeon always increasing in difficulty, called Shattered Realm (only available from Forgotten Gods). When I multiplayer, I always play it, its really fun! And challenging 
Maybe go to Ultimate (can skip Elite now) after killing
It doesn’t take long to do that especially when you melt everything on Veteran. Skip side quests
On Ultimate try killing Ravager, Crate and Callagadra superbosses.
Grim League post season version should have a lot of difficult challenges, scalable rogue dungeons and even more superbosses.