I used to be a stronger proponent for keeping anemones out of the same tank, but the problem is I'm a hypocrite when I suggest that.
Ultimately, I still do think the theory of keeping them in distinct setups is better overall. However, there are plenty of examples of multiple individuals in the same system seemingly without problems, but I feel there is more to anemones than we generally give them credit for -- the potential for problems is there - but whether that "potential" becomes "reality" depends on probably more variables than we can predict. In most cases it's probably not a problem at all unless one of them is stressed for whatever reason, in which case it could become more susceptible to whatever it is.
I used to keep my H. magnifica in with my BTA's. Of course with BTA's they split. When it was just the two anemones, there was no problem. But then the BTA split and I had three (2 BTA 1 ritteri). Still no problem. Then another split. Now 3 vs 1. Then another split. Now 4 vs. 1. Suddenly the ritteri wouldn't inflate as much. It got worse over several days and finally was not inflating at all. I thought I would lose it. I started pulling out the BTA's. Within a week of the separation, the ritteri started looking better. What exactly happened I'll likely never know, but it seems suggestive to me that the colonial nature of the multiple BTA's tipped the balance.
So .. having said this, I have one tank with 3 BTA's and a S. gigantea carpet. The BTA's are 2 of one kind and one rose. There don't appear to be problems yet, but I'm planning on keeping them in separate system in the future (for one, having a tank full of anemone like that really restricts what else you can keep in there, they take up a lot of real estate!).
I also have another S. gigantea carpet and it was housed in the same system as my H. mag with no apparent problems. I have the H. mag in its own tank now though and hope to keep it that way from now on.
Hope my rambling makes some sense.