Whenever I'm asked why I need more than one bike I ask them why they need more than one pair of shoes.

Anyway, I have separate bags for each bike. My commuter has the biggest bag because I like carry two tubes and a patch kit and a multi-tool and a couple of chain links and several zipties and CO2. I keep my lock at the rack at work. I'm more likely to be late for work and by myself on the commuter. On the other hand, my racie bike is more minimal: one tube, multi-tool, one CO2. I'm usually on a group ride or race with that bike so I'm not likely to be stranded.