We can both play Brucepack mod singleplayer just fine, and we can load the mod successfully on both ends in the character select, but the instant we start playing the chat gets FLOODED with something along the lines of "fish.dodgeroll.gml -> (<random numbers="">) Player does not have variable dodgeroll", bringing performance down and preventing the mods from kicking in.</random>
That is a mod-specific issue - likely the mod simply wasn't tested in coop, and therefore does not work correctly. You'd need to ask author about that.
Also Player 1's hud seems to be misaligned in co-op but not sure what's causing that. Saw there's a mod that lets you re-arrange HUD elements anyway so if we get this working we'll try that.
That is a known issue and will be fixed in the next update - is a mishap due to changing something in GUI drawing logic.