What would be wrong with mounting it high on a rear rack? One of my bikes has a big milk crate on the rear rack- if you rested bike #2's top tube along the top of the crate (so it was perpendicular to bike #1) and tied it down there, I don't see any major problems that could arise. I mean, you would have to be real careful about your balance, but otherwise it would be just like an ordinary car rack.