I've done what you are describing. A couple things to think about: 1. Not all 5th wheels are balanced the same. My current 5th wheel (30 ft) has the axles further forward than my previous one. I double towed with the old one and had no problems, but I don't hink I would dare do it with the new one because of tounge weight. 2. You are only legal to 65'. You can buy a "long load" permit that makes you legal up to 85-90 feet (I think that was the numbers) - My dodge 4 dr short bed + 30 ft 5th wheel + 18 ft trailer put me at about 67 feet. It costs something around $150-$200/per year to buy the permit. 3. I wired two brake controllers as Marc suggested, and it worked great - I only hooked up the manual slide part of the second controller (for the second trailer) - so I could manually activate the second trailers brakes, in addition to both trailers brakes working when I pushed on the brakes. (You have to use a diode for the second controllers wireing). If the second trailer started to move around a little, I could just slide the button over on the second controller, and it would straighten right out.
I have also flat towed the jeep a few times behind the 5th wheel. I think this is a better way to go. There was no sway, and the 5th wheel isn't carrying the weight of the jeep/trailer. The negatives are no brakes, tire/drivetrain wear on jeep. (and you can't see the jeep - unless you install a backup camera)
I would think long and hard about doing it - People do it and are fine, and others have crashed and been killed.....
If you have any other questions - let me know.
Kirk