lol never had axolotls, don't know, but sometimes they sit on each other because they obviously think it is a branch lol for those who'd think they might be mating- they are not, not yet anyway.

my background just regular exo terra foam one. I don't think it matters what kind of branches you should use, as long as they are different in diameter/ shape and smooth it should be fine. but whatever you're doing make sure they're strong, same goes to your water bowl, they gotta be stable, they like to sit on a edge and might flip the whole thing on themselves.