I have wondered the same, since I'd love to take my bike on some of our trips. I know I don't want to add 650 lbs of weight to the back end of the frame. The bouncing at the end of that long moment arm must have negative effects over time. But I wouldn't be too concerned about pin weight, since it would actually reduce pin weight, being that it is behind the trailer axles. And I also know I don't want to tandem tow 2 trailers.
Well, on my recent trip to Gatlinburg, I saw a 5er drive through town with a motorcycle behind it. The owner had a small cradle type device attached to the 5er receiver hitch. The motorcycle front wheel rested in the cradle and its back wheel was on the road. It happened too quick to see how the bike was kept "straight", but there must have been some way to do so. It looked like a great idea. My only concern would be how much bouncing and wear was happening to the bikes rear end and rear tire.
But it seemed like a great idea - minimal weight on the back of the 5er frame, zero added pin weight, no issues with 2 trailer pivot points, and it looked easy to connect/disconnect. Of course, it would add to GCVWR, so you'd need to make sure you stay under 22K total.
I just wish I could have caught up with the guy and asked where he got it. A quick Google search showed me one similar (though not as "beefy") cradle.
http://auto.howstuffworks.com/auto-p...-trailers3.htm
Let me know if anyone has info on such a device.