I take it you mean the rotating pedelec sensor? These fit onto one side or the other of the bottom bracket, quite easy to do. With a tandem the best place is probably on the front bracket, this will dictate where the controller and battery will be located unless you can extend the wires.
The cables from battery to controller to motor should be kept as short as practically possible. Normally the battery to controller with be easy as these two items tend to be in close proximity of each other. The motor cable length will depend on where you intend to mount the battery and controller and if its front or rear wheel drive.