I’ve never found the borderlands games to be particularly suited to multiplayer. I played a lot of BL2 and TPS with a couple of friends, and it was fun, but it was rarely as fun as playing solo.
So many things about the games just don’t work well with co-op. Like the fact that players want to frequently check their inventory or respec, which is boring for everyone else. Or that it gets too easy when there’s always someone to revive you. Or that you can’t really soak in the (very well crafted) atmosphere as well. Or that one person wants to experiment, or go for particular challenges, or farm a particular mini-boss, while another just wants to complete main missions. Or that vehicles suddenly get weird and laggy when in co-op.