All male plecos are territorial. If you have sufficient space and and well spaced caves for each male, they can co-exist in multiples in a tank. They each need their own territory and cave to do so. I doubt you should have any problems keeping two males in 55. That said, there are no universals when it comes to fish and only time will tell if you happen to have two that simply cant be together. it does happen. The more cover in the form of wood, rocks and plants you have, the easier it is for them to cohabit peacefully. Besure to put their food in so they each have some near their cave which will mitigate any arguements over food.