I have 3 pairs in my 300. A Saddleback pair in a carpet anemone that live in the end of the penninsula, an ocellaris pair in a GBTA that live in the middle, and a maroon clown pair that live in a RBTA at the other end. They have all been in there for at least 4 years with no issues. They all eat together with the rest of the fish, then go back to their "spot" to hang out. Maybe its the other fish in the tank that range from 1" to over a foot that keep them from getting agressive. No idea.
It can be done but is uncommon at the least. Probably in your case because they have their own home they feel secure. However my friend with the 550 had it set up with each having their own anemone and it did not work. Of course one pair was GSM. And tank size clearly is an issue. Larger (longer) tanks increase the likelihood of it working.