We are owners of one of the (relatively) few dual disc tandems out there - a dual Avid mechanical disc equipped custom Bushnell, born in Dec 2003.
Until I put a few thousand kilometers on it in all sorts of conditions with all sorts of team weights on it, I can't give reports that are more than first impressions at this time. I can say that the brakes stop durn good.
Currently almost all my riding is as a very light team - 250 pounds (my son and I). I'm quite sure I could do an endo and catapult my son over my head if I chose to.
Brake lever travel is "different" (we are using Shimano STI levers) than with traditional brakes. Much of the stopping force occurs after a great deal of lever travel (at least with the STI), however, with very little lever effort. Some might find them mushy. Modulating the brake is possible, but requires a lighter touch and a little practice as compared to traditional brakes. After a ride or two I do not notice this issue.
Its possible this may be seen as a benefit for the hand-strength challenged. I'm not one of those, but certainly I feel I can exert more stopping force from a hand position on top of the brake hoods than I can with my road bike.
Stopping force does not appear to be a problem but will comment on this more once my wife and I get some real time on the bike - then we'll be a 350 pound team. Vancouver has plenty of steep hills, including one straight out my front door - our house is quite literally the highest house in the city proper.
I am already sold on disc brakes for use in the rain - one of the main reasons i wanted them was the rainy weather we get here in Vancouver does not stop us from riding (both commutting and recreational). Also the bike seems to remain cleaner - a minor but not unimportant benefit as far as I am concerned.
Our Bushnell, like Ericksen dual disc tandems, has the front disc brake mounted on the right hand side of the fork. I plan on doing some tests in my basement, with the skewers completely loosened, to prove to myself that this mounting practice completely does away with the issues reported by others with left-hand mounting and drop out failure.
I've not noticed any significant steering torque issues even under hard braking.
I've not found the beefy fork contributes to a harsh ride at all - quite the contrary, this aluminum tandem is nicely responsive yet more comfortable than all of our other bikes, at least speaking as the captain. Perhaps I should have my son speak up
A more detailed report will come once I can get my wife on board for an extended ride.