i have a sigma wireless that is very easy to install
no need to deal with wires
i think most sigma wired ones can be converted to wireless with a kit
or install the wired on one bike and wireless on a second bike
then all you do is move the screen from bike to bike with the easy twist off motion that iluvtechnical refers to

the caveat with the wireless is changing the battery. not that it's hard to do, but my battery died and i have yet to change it!

get a garmin gps with heartrate, cadence, altimeter, mapping, driving directions, satellite tv, karaoke, internet, cell phone, twitter, playstation, bluray...

otherwise get this for $10