Originally Posted by TandemGeek
There are just a few and they're rather expensive.
Debbie has a wireless Polar S720i which now sell for between $250 and $310 which can be modified to boost the transmission range such that it will transmit from the rear seat stay the the stoker bars. Works great, but if you don't need, want, or use all of the other functions you're spending about $225 - $295 more than you need to for speed and distance measurement.
Cateye also makes a Dual Wireless and Dual Wireless HRM computer that retail for between $129 - $179, but that you can also find on Ebay for about $89. The problem with these is, there were several generations of the computers made until they got it right. I have no idea which ones are being blown out, but I know a lot of the early buyers ended up sending the things back a few times before giving up on the product. However, there are also a few teams using them that haven't had (or at least mentioned) problems.
FWIW, I run a Ciclosports HAC4 on the front of our tandems and there are no interference problems with Debbie's S720i.
We use the Shimano Wireless Flight Deck (I wanted the graphical gearing display) and Polar heart rate monitor for the captain and Garmin Forerunner 305 GPS providing speed, distance, average speed and heart rate for the stoker. I looked at different solutions of wired and wireless computers to get bike and human performance metrics (downloadable to a computer), gearing configuration and heart rate for both parties. This solution is reliable and does not require running wires or worrying about distances or interference between wireless devices. Santana installed the Flight Deck from the factory and I bought the Garmin on Ebay and the Polar at the Bike Shop with a discount. Total cost of the solution was $129 Flight Deck (installed), $249 Garmin (wraps around the stoker handle bar) and $90 for the Polar F5 – total was approximately $500 including tax. Heart rate data and bike performance metrics are downloaded to our PC and we track our routes, distance, average speed, elevation gained and heart rate using the Garmin and Motionbased software as well as Polar's HR performance website.