I've seriously thought about buying one. I want to haul my YJ (say 5000 lbs) and tent trailer (say 1500 lbs--{I think that's WAY generous but I like to build for worst case scenario}) around and that'd be a decent load. I'd like to build it for two rigs but don't know how often I'd use that feature. I'm 100% sure you'd want brakes on both axles, a weight distributing hitch and a perfectly positioned load to travel at safe freeway speeds with two trail rigs on it. Not that it couldn't be done, but it'd be interesting from the bumper pull.
There's TONS of goosneck trailers out there that will haul 7000 and 10,000 lb payloads. I'm thinking that's the way to go. To haul two rigs regularly, I'd try to get a 12,000 lb or similar trailer. Safety margin is a good thing, IMHO.
My money is on doing it 'right'. I'd rather invest in the gooseneck setup.