I know in general that most of you are talking Road bikes, and I have a mountain Tandem. This was initially fitted with "V" brakes and these are quite adequate for the majority of users. However, I found that on the longer excursions that I take, 50 to 100 miles, and totally off road, Brake fatigue was coming in. This was both on the pilots hands, and the extra pressure to get gritty blocks to work on muddy rims. 4 Months ago we changed to Hydraulic Discs, Front and rear and what a difference. We now use less brake altogether, (With "v"s a confidence test for them was always given before full application), Downhill speed is higher, braking period is shorter, and we have stopped wearing out rims.
I can assure you that these Hydraulic beasties are far more efficient than the cable operated things I tried about 5 years ago on a solo. They were about as efficient as putting a stick in the spokes. They did not work unless you put on full pressure, and then they suddenly stopped the wheel. Not confidence inspiring at all. I would still have reservations about using any cable operated disc system, but if it is possible for any of you to try a hydraullic system, then you will find the near ultimate in Tandem braking that it is possible to get.
Incidentally, the system I fitted was the new Hope Mono M4 with 200mm discs, and boy, do they work.