Longtails with wideloader type bars seem to me to be the simplest/easiest way to haul another bike. I do it regularly with my Yuba Mundo. The front tire of the bike to be hauled is secured between the bike frame and the wideloader bar and the headtube or some other part of the towed bike is bungeed to the longtail.

I use an old tube tied between the wideloader and the frame and slide it back or forth as needed to support the tire of the towed bike. A 700C wheel fits like it was made for the Mundo wideloader, but a smaller wheel needs the old tire tube to keep it from falling down to far.