To the OP: I'm not clear if you have disc brakes front and rear, or rear only. (One of your later posts refers to two rotors that came with the bike. One is shown mounted on a rear wheel but the other, is it on the front wheel or is it a spare rear without a cassette?) Reason I'm asking is that no rear brake contributes very much to stopping a vehicle. Even on a tandem the weight shifts forward during deceleration and the front tire still does most of the stopping. So if you have only a rear disc brake it won't stop any better than a V-brake, as you note. If it stops as well as your old V-brake, it may already be optimized. The chief appeal of disc brakes for tandems is that they work a little better than rim brakes in the rain (more predictably at least) and avoid the hazard of melting an inner tube or plastic rim strip from prolonged downhill braking. (And as a way of getting rid of V-brakes!) They don't stop any faster than short-reach rim brakes in dry conditions in my experience. We use two short-reach dual-pivot rim brakes for all our riding locally. We are light and our hills are steep but short. We brake hard & often enough to wear out front pads but not for long enough at once to overheat a rim. For mountain touring we use BB7 discs front and rear for the reasons alluded to. (Of course not all bikes can use short-reach rim brakes, which is why cantilevers were invented.)
So, if you have discs front and rear, my next question is, Does the front work well, or does it suffer the same complaints? If you are getting poor performance from a front disc, than you really have a tougher problem because sag-weight of cable, friction, return spring force, all of those fixable issues that plague rear brakes should not be operating up front. If the front works well but the rear is disappointing by comparison, try the excellent tweaks suggested by the other posters -- we've used 'em all, and they do improve things -- but in the end you may be satisfied if you think of the rear disc as a device for augmenting heat dissipation (though not as good at that as a drum brake so you need to use the front, too) and the front as your actual STOP! -ing device. (Yes I know that all braking is heat-dissipation; I'm just thinking about which lever to grab harder when a car turns left in front of you.) I don't think any rear brake will slow a tandem down much if applied alone, without the front. I hope I'm not stating what's already obvious to you, since you cross a lot more contour lines in a day than we do....