This summer I was downhilling in Åre Bike park, in Sweden, and there was a married couple riding downhill on a tandem. I believe it was a Nicolai (not sure though) speced with a Boxxer Team up front, Rohloff hub and a pair of Hope M6 brakes. I talked to them a bit and he said the major problem was not the downhilling itself, at least not on friendly DH-tracks where high ground clearance isn't needed. The problem was braking. Many of you who have been to places such as Whistler know the pain you get in your forearms. This dude claimed it was faaar worse stopping "him+her+50lb" bike.