I've tried a few different computers, but I think for me the ultimate computer is one of the Planet Bike ones. They're dirt cheap ($16 for the basic unit) and probably the most durable computer on the market. Plus you can operate it wearing lobster claws. The 5.0 is good for basic functionality, and since I have only one bike, it's probably the one I'd get if doing it again. I got the 9.0 because the thermometer seemed like a fun toy.

Many computers support multiple computers. The Sigma line actually auto-switches from bike 1 to bike 2 just by moving it between bikes (the mounts for bike 1 and bike 2 are slightly different, so it can tell which bike it's on).

The computers that I've used that support two bikes keep a separate odometer for each bike, plus a total mileage odometer.