I purchased a VDO C2DS wireless cyclecomputer this weekend becaused it claimed 1.5 meter range. My thinking was it should be great for the stoker, which would be at the typical limit of 0.7 meter range of other brands. The VDO uses digital encoding instead of analog over the RF link between transmitter and computer.
Worked great! Reliable speed reading with stoker on the tandem.
I installed the transmitter/sensor near the rear brake, the two cable ties provided worked fine. The sensor magnet is near the rim, but is very small, so no problems mounting. The computer was installed on the stoker's handlebars.
I use an Shimano flightdeck for the captain and didn't have any interference between the two wireless systems.
The VDO C2DS was on sale at Performance Bike for $50. They also make three other wireless models that use the same digital wireless technology.