Our old bike has a front-wheel speed only Cateye computer for the captain.
For the stoker we use a Wahoo Bluetooth Smart speed and cadence sensor (half price at a major homewares retailer, by lucky chance) and my new Motorola phone which runs Android and is Bluetooth Smart compatible, plus the free Wahoo app. The phone doesn't pick up the sensor signal instantaneously like with a Cateye or Sigma computer but it is connected after about 4 secs.
Still working on a mount for the the phone on the stoker bars, in the meantime Mrs WPH can read the screen thru my jersey pocket - we have been riding, when possible, after dark. For the first time she can see cadence, which is a help for my poor old knees. And we could upload the GPS track to Strava or whatever if we wanted.
The phone was AUD170, the sensor was AUD40.
So many options and solutions these days, no doubt wireless makes it all much much easier.