Get a seat tube rack for the road bike. The Bug is fine to wear on short walks around campus/work but if I had to walk a longer distance (>1 mile) I would definitely opt for something with a hip belt and sternum strap. And on the bike that goes double I think.
I use a Bug with a rack (the Bug is great on a rack OR on your back walking), but a cycling specific Deuter Trans Alpine 25l backpack when on my bikes with no rack. It is much more comfortable than the Bug while riding as it is designed for cyclists.