I have 2 pair in my 120 with no problems. They were all purchased when they were small and added within 2 weeks of each other. 1 pair percs and the other pair black ocellaris. There aren't any nems in the tank. Occasionally they hang out in the various LPS corals but most of the time they dig a hole in the front corner and 3 of the 4 hang out there together.
I wouldn't attempt this -- a 120 still isn't all that big.
It may work in the short term, but once one of the pairs starts to spawn, all bets are off. I had a pair of mated pink skunks that were very docile at first -- once they started spawning, all things changed. They became super aggressive, to the point where my dwarf angels weren't safe -- ended up selling them to a local breeder. I don't even want to imagine what they would have done to another pair of clowns.