I have pulled doubles before and so I think that I may be able to offer some thoughts for you. First, I would bet that it would take a fair amount of modifications to your travel trailer so that it would be able to handle the stress and weight of your flat bed. I know this because I had to do this for a friend of mine. Second, while it is legal to tow two bumper pulls in Utah (or so I was told) it is not legal in other states. Third, even with a load distrubuting hitch how much can your hitch on your tow rig really handle. If it is a class four (I will bet that it is) then it can only handle about 10,000 or 12,000 lbs max. If your trailer weighs 6,000 (again I am guessing it does empty) then you can only handle 4,000 to 6,000 more total. Flat bed trailer and crawler together.
When I hauled doubles I had to do it with a gooseneck flat bed (rated at 20,000 lbs.) and then a pop up camper behind it. Total weight was at 15,000 lbs. 7,000 for the flat bed, 5,000 for the off road toy, and 3,000 for the camp trailer (it was fully self contained). And the only way to do this is with a one ton dually, diesel or larger! I was also a little too long for the law at 68 feet in length (New Mexico states 65 feet max).
LT.