Actually, I've been thinking about how to do the same thing. Securing the front wheel was the problem I kept running into. Then I thought, take the wheel off, and all you have to secure are the forks. And they make stuff for that!

I'm thinking I could mount one of these on my rear rack. If it's secure enough for a roof rack, it's certainly secure enough for a rear rack on a bike. As for the towed bike's front wheel, all I've come up with so far is to zip-tie it to the towed bike's frame. I suppose that in a pinch you could just hang it from the towed bike's bars.

Anyway, that's where I've been heading with the idea. Won't have to try it for a couple of months, so I don't know exactly how well it will work. But for $20, it's worth a shot.