Were using 2 VDO C3DS one on Capt.. and one on the stoker both using one sensor the speed is mount on rear wheel and cadence on rear stoker crank. The transmitting distance for the VDO C3DS is about 15ft so range is no problem and they have no interference that we have ever seen working flawlessly.
I'm also using a slightly larger magnet on the crank for cadence so the sensor picks it up better.