In my experience, any time my RBTA and my Sebea touched, the sebea got stung. It was never enough to cause any kind of drastic changes, but they definitely couldn't be in close quarters. Given that they were both 18" to 24" in diameter, I'd say only one in a 24 gallon tank. Big problem with anenomes is that you can't usually trust them to stay put, so all your other corals are at risk. My RBTA is in half of my fuge now with some rock and his tomato clown buddy.