Nice, seems my idea isn't as original as I thought! Even if they do occasionally try and maim you, bungees are wonderful things...

For the record, the execution of my plan worked perfectly, and once the bikes were separated resulted in a great day's cycling in the countryside with my gf - another convert to bikes! The setup was a little wobbly during pothole-avoidance, but surprisingly stable at speeds of ~20 mph or so - far faster than I'd be willing to bomb along on the small-framed second bike on its own (not to mention I look like a clown trying to ride it, yet it's still somehow too large for my gf...)