Have been using separate VDO wireless digital computers on three bikes for several years. Disadvantage of wireless is that batteries don't last as long as wired and with three computers, seems like they are always dying. Very long wires would be needed for my recumbent bikes - with wireless, only the digital ones have enough range. Sending units on the forks sometimes get knocked askew, too, of course.
Very close to moving to a single very basic Garmin GPS unit to share between the bikes. I track mileage for each bike by entering into a spreadsheet - that shouldn't be a problem with the Garmin. Might even be easier, what with downloading data to a website. If I understand right.