I had 2 large ones together that were awesome. They swam together and mirrored each other constantly. I ended up selling the small one (16") and put a big beefy 22" one in it's place. I had to get rid of the big guy because they constantly beat the hell out of each other. So in my experience it's hit and miss, but it can definitely work.
I put 4 in my 15k in the late 90's. They quarrelled very seldom. Over the years 2 passed away. The 2 that are left I have never seen fighting with each other or anybody else. They are in the 15k videos.