going down big hills is a special case so far as the brakes are concerned and it is one that many bicycle disc brakes struggle with, even on solo bikes. Even a twisty 1000' descent (think of the south downs rather than the alps) is enough to boil the brake fluid in some hydraulic systems and/or get discs hot enough to turn blue-coloured, and that is on a solo bike. On loaded touring bikes and tandem bicycles pretty much anything can happen.
About 20-odd years ago it was proposed that a standard procedure was employed for testing bicycle disc brakes. This was equivalent to dragging the brakes down a descent, and was modelled on a real descent down a real mountain road in the USA. None of the then current brakes were able to pass this test. The result of this testing was not that they then went and made better brakes, but that they arbitrarily revised the test so that it was about half as hard....
. That brakes might pass this test should be of scant comfort to those who ride up and down mountains; there are real roads that you can ride where your disc brakes will probably not be good enough to allow you to descend in safety. This goes double for loaded solos, tandems, etc.
It might be worth giving some attention to a comparison with other (say motorcycle) brakes. A fully loaded motorcycle might comprise the motorbike itself (~200kg), two passengers (~180kg) plus their luggage (~30kg). The brakes fitted are not much more than 'good enough' to allow a long descent to be negotiated in safety (NB the brakes are usually saved by engine braking but you cannot rely upon that). Thus a realistic target for a loaded tandem bicycle (which weighs a little over half as much, say) might be to have brakes that are half as good in terms of their power and thermal capacity. You don't need to be an engineering genius to see that bicycle brakes are likely to be not much more than ~1/4 as good as motorcycle brakes, i.e. about half as good as they should be for tandem use.
Thus tandem bicycles which are sold with just two disc brakes might stop OK in normal use but they are liable to be under-braked when it comes to any kind of long-ish descent where you are likely to drag the brakes on the way down.
BTW I'm not saying that two rim brakes would be better; it takes real skill to safely negotiate hilly terrain on a tandem with just two brakes of any kind. The saving grace in traditional touring tandems has always been the drag brake; if used correctly this usually gives fair warning when it starts to get too hot (smells, smoke and/or fading) and there should be enough in the other two brakes to safely bring the machine to a halt so that everything can be allowed to cool off for a few minutes. Drag brakes normally recover from such episodes without permanent damage arising. To date I've not seen a tandem with a traditional drum brake fitted as well as a rear disc (which ought to be possible with revised mountings) only a rim brake and a disc brake at the rear. However if a rear disc brake is used as a drag brake then there is (regardless of disc size it seems) a pretty fair chance of parts in the brake overheating and causing problems. Overheating problems in disc brakes may (unlike drag brakes which are drum-based) cause problems of a permanent and/or sudden and unforgiving nature.
So in a nutshell, you cannot take it for granted that your (two) disc brakes are going to work OK on big descents that require brake dragging; on a loaded solo it is of real concern and on a tandem bicycle it is very doubtful that the brakes will be good enough unless special (pre-emptive) care is taken. Unlike drum brakes and even rim brakes, the indications that disc brakes are getting too hot are not as obvious and in some cases (such as hydraulic fluid boiling) the brakes can work normally one pull and there can be nothing the next pull, which is incredibly dangerous.