I had a gadget years ago that we built. It clamped to my rear rack and hung down a bit. It was similar to the bed rail fork mounts that you buy today. Then the front wheel was bungied to the frame of the rear bike. Worked pretty good, but got stolen a while back and I have never bothered to replace it. Handling was affected but as long as you watched where you were riding and kept the speeds sensible it worked.

Aaron