I have a common pleco with another (might be common but looks a lot like an L21) and the common pleco is very territorial; basically dominating the piece of driftwood in the tank and chasing away the other violently if is comes around there. The other stays in the sword plant I have.
Eventually they will be separated once I get some more tank space, but I have another common pleco growing in a smalled tank. I'll probably end up giving one back to the LFS.