I run Garmin Edge up front and I have a wired computer for my stoker. I wouldn't assume one sensor battery would be able to drive two computers.

I didn't know whether the signal of a wireless computer would reach the stoker handlebars from either wheel. The instructions for most of them say that the two parts should be less than 18 inches apart.

I did have to splice an extra length of wire so the stoker computer would reach the rear wheel. I soldered it and used heat-shrink wrapped sleeves to weather proof it.