2 is not likely to work out unless they are a pair. The dominant one is likely to kill the subdominant one, with nothing to distract him. 3 or 4 with some dithers would be a better bet, and I woud go for something in the 200+ gallon range for this. Of course, if a pair forms, they could possibly kill the others.
SIngle RD in a 90-125 gallon tank would be fine imo, and would be much easier to do.