All else being equal, V-brakes or cantilever brakes are better suited for use on a tandem than disc brakes. In my tandeming experience, the maintenance and adjustment intervals are much longer with rim brakes than discs.

Normal braking power is about the same with properly set up V-brakes, cantilever brakes, or cable-actuated disc brakes (assuming 200or 203mm rotors). Extended braking on long descents challenges most braking systems. Without a third brake, the best option for long descents is probably V-brakes with *heavy* aluminum rims. You can read several threads about tandems overheating brakes.

In my mind, the advantages to using disc brakes are:
-more rim options -- especially carbon rims, really wide rims, or aero rims
-multiple tire sizes on the same bike -- i.e. for swapping between 700c x 32mm tires and 650b x 52mm tires
-rims stay cleaner
-rims last much longer

In my mind, the disadvantages to disc brakes are:
-higher upfront cost
-frequent rotor truing
-you can expect to hear "tink tink tink" for 30 seconds after most stops