i sent the PM originally because my post ended up being quite long. here is what i sent chefpels:
i did it for one summer in colorado, take this picture and insert pop-up between truck and raft. http://www.mountainbuzz.com/forums/members/609-albums385-picture1875.jpg
my set up was a f150, a fleetwood taos pop-up (8 x 6 box, the smallest size), and a 4 x 10 utility trailer with 15ft raft. it is legal to do in colorado, and i passed several cops on my summer trips that did not pull me over. one requirement in colorado is that the middle trailer (pop up in my case) has to have trailer brakes.
i had to build the rear receiver hitch for my pop-up, no hitch shops would install one and the estimate i got from a local welder was $700. i pieced it together with heavy angle iron from a local steel shop and a HF bolt on bumper receiver hitch. i spent about $200 on my hitch.
one thing you need to be very conscious of is the loading on the raft trailer to maintain reasonable tongue weight, but not over loading the raft trailer to cause a decrease of the pop-up tongue weight. theoretically, if you put 200# on the raft trailer tongue, the lever action thru the pop-up axle will decrease the pop-up tongue by the same amount (more/less). for my set up, i found the tongue weight of my pop-up was around 300 +/-, while the tongue weight of my raft trailer was less than 100, so there was still good tongue weight on the pop-up. the trailer i put my raft on is extremely light weight and seemed to work well for this set up, but i think some heavier utility trailers might be a problem. i was told that main objective is to always maintain significant tongue weight on the middle trailer, and that will help minimize sway in both trailers.
the only time i would see any issues with sway on the raft trailer was on long down hill stretches with lots of curves (like on vail pass or down from eisenhower), and if the pop-up got bouncing a little. my guess is that the bouncing of the pop-up would zero out the tongue weight of the raft trailer, and thus allow it to sway easily. i would handle this by keeping my speed relatively low around 50-55 on long down hills with curves. other than that, i could travel at typical highway speeds on the interstate at 65-70 with no issues.
for lights, i put a combo RV/4 pin connector on my truck, and then ran a 4 wire kit under my pop-up and zip tied it to the frame. when pulling both trailers, i just plugged them both into the combo connector on the truck and both sets of lights worked fine.
ultimately, my wife did not like towing both trailers. we sold the pop-up and bought a van to camp with instead, so now we can just pull the raft behind that. good luck with your set up. later.