hi merinextralight
may i ask what rotors and pads you use ?
our tandem is set up similar to yours, although with a titanium frame (cycleschinook)
we use the red pads for the front and the blue ones for the rear brake combined with 180 mm swisstop rotors;
last weekend we had to descend 10km/1000m on a fireroad, so i had to brake all the way (took us 25 minutes)–there was enough braking power, no fading ;
for the road setup we use shimano xt rotors, which are fine, but from feel i prefer the swisstop ones
thanks andreas