I've got a 14 ft trailer I haul my 500 on. When I bought it I thought I could haul the 500 and my 2007 420 Rancher on it. It likes about 6 inches being long enough to load both and close the tailgate. Mine has the 4ft high tailgate that doubles as a loading ramp. A trailer without a tailgate might work, but the 500 and Rancher would still have to be touching, I'd think, to get them both on. You really need a 16 ft trailer to haul both, unless you have a trailer wide enough to load the Rancher on sideways.
I've thought about building a set of ramps on the front of the trailer to run the Rancher up on, to where it's front wheels would be on top of the front rail of the trailes to get the 500 on behind it. Haven't tried it yet though.