What sort of riding do you intend to do? How heavy is your team?
The DMR fork is exactly what I was looking for when I had stopping issues with my KHS Sportmania - a nice fork to which one could mount a disc brake. Instead I got a POS 50 mm suspension fork...and that lead to a round of upgrades which culminated in a Marzocci DJ, 20 mm Chub hub and Magura Gustav (all of which somehow found there way onto my son's MTB bike...not sure how that happened). My riding of the KHS changed too. Instead of riding with my oldest daughter from one subdivision to another for soccer, my son and I were riding singletrack in the mountains.
I will probably get a DMR fork when the 3rd daughter is big enough to ride the tandem...
Depending on what axel to crown distance you are looking for the rigid Funn fork is another good option. It has clearance for the fattest 29er tires, 20mm axle, is disk compatable and is plenty strong for MTB tandeming. The A-to-C distance is 19". The cantilever bosses are for a 24" wheel or something weird so they are pretty useless.
Thanks for everyone's input. Comments posted here by Tandem Geek & Prairie Boy opened my eyes to the expenses I was facing, and I've since regrouped.
The initial frame and fork have come and gone, never having made it out of the box. In its place is a decent used Cannondale tandem with fork, wheel and brake upgrades underway. A bit more than I wanted to spend, but we're getting a great bike out of it.