I've succesfully kept a tomato and a false perc. Here's how it went...
Many years ago (over 10 years) when I got into saltwater, the second fish I got (after a pair of yellow-tail damsels) was a false perc. I tried a flame angel after that, but he didn't make it. I wanted a red fish, so I got a tomato clown. That was in a 20 gallon tank. I soon moved up to a 30, then a 50, and eventually scaled back down to a 29, and still had the two clowns. Last December, I suffered a prolonged power outage and lost the false perc, who I'd had with the tomato for 10+ years. The tomato survived, and within a couple of weeks I got another false perc. The tomato and the new perc do fine together.
My experience is probably the exception to the rule, but it's probably not that unusual to keep two different clowns together. I think not ever having had an anemone helps. That way, the two clowns have never really had something to fight over.