I'm getting a new exhaust done on the RRC and was wondering if it's necessary to run two CAT's? I was going to run just one CAT and one muffler.

Probably not necessary...but I think most V8's run 2 cats to get each cat closer to the header...more heat = better function :confused:

That's my understanding, but I could be wrong. I'd imagine that if the shop you are having do the work is well versed on exhausts, then they should be able to tell you whether 1 cat will suffice.
Last but not least, make sure the 1 cat you are using will flow enough to not bottle-neck your exhaust (which is probably a better reason why the factory runs 2 as stock than the heat explanation).