I have even heard of 2 pairs fighting in a larger than 300gal before.
I used to ask the question about 10+ clowns in a 30 or so gal at your LFS and why they don't fight. I guess you can pull it off when they are still young. Once they find their significant other, it doesn't work.
I really wanted a pair of "regulars" and blacks in my 180 till I learned about it. I don't have any clowns in there yet, but when I do, I think I'm going to try 1 black and 1 "regular (orange)". I hear that it's possible if they are from the same family (ocellaris). I also heard that there is a VERY slim chance they will mate, but they will pair.
I hope that helps in deciding which type you want.