Actually, while there is some degree of truth to the ease with which discs could be adapted to the rear wheel of a tandem vs. front forks, the real driver was addressing rear tire blow-offs from overheated rear rims/tubes/tires. Yes, true stopping power does come from the front brake, but human nature being what it is, tandem captains tend to use overuse the rear brake to control downhill speeds and that is why almost all of the tire blow-offs that you see on tandems are at the rear tire. So, that is why the discs first appeared on the back of road tandems instead of the front.

Moreover, as Galen noted, discs are not universally looked at as a proven product for use on road bikes or tandems. Off-road, their utility and value are unquestioned but the demands of road riding, and particularly the demands put on brakes by a tandem are valid reasons for proceeding cautiously with their use. Additionally, it truly does take someone who has gained experience using the disc to help guide decisions regarding which teams might benefit from them vs. those who would do better with more conventional brake configurations. The latter is still a very scary area as even some of the tandem specialty dealers dont' have a lot of experience with disc brake systems, never mind a LBS dealer who hardly knows how to spell (or sell) tandems.

Bottom Line: Yes, from an engineering perspective, you would definitely want to put the disc on the front; unfortunately, technology acceptance and human behaviour dictated a different migration path. However, the logic does support the idea that dual-discs may have great promise for use on tandems... but only time will tell.