TandemGeek

I'm not sure how you get to a factor of 3x (I'd think it's somewhere between 1.5x - 2x), but it's a moot point if the fork is robust enough for long-term use on a tandem and the disc's use doesn't create any brake torque steer effect. Those are the real concerns and, in fact, most well-executed front disc implementations on single and tandem road and off-road bicycles more that address those issues.

Again, there have been exceptions / problems with some poorly designed disc brake fork adaptations relative to drop-out orientation (See aforementioned articles by James Annan). But, excluding those exceptions, I can tell you I've never experienced any brake torque on our off-road tandem which is fitted with a very robust, 4-pot downhill hydraulic disc and 203mm rotor. Now, to be fair, the hub and fork interface is a lot more robust than your average road tandem with a 9mm steel quick release skewer. But, that said, having ridden an Erickson road tandem solo with a front disc and spent time at rallies riding with about a dozen folks who own front disc equipped road tandems, I did not note nor did any of them make note of having any brake torque steering issues or a left-fork failure / fatigue issues.

There are all kinds of examples of asymmetric brake and drive train designs to be found on bicycles and motorcycles, yet few if any problems when the designs are sound and well executed in the manufacturing process. Motorcycles have for years used asymmetric, single front disc brakes as well as single sided rear swing arms for chain and shaft drives. Now, you can certainly take things a bit too far as Cannondale did with their disc-equipped Lefty single leg forks (e.g., Raven with lefty), which came back to bite them... but it was a durability issue not a steering / control problem caused by brake torque steering.

Bottom Line: Front discs work just fine on road and off-road tandems so long as the forks and other critical components are designed for use with a disc brake.