Here's probably the most irritating story about disc brakes and tandems that happened to me here in the 1990's.
In 1998, we put together a 25th anniversary model tandem with a special head badge and full custom everything. There were lots of rumors floating around about 'tandem worthy' cable actuated disc brakes (hydraulic disc brakes don't work on travel tandems) in the tandem world, but to that point I hadn't seen anything that was good enough for a road tandem.
One of our 25th anniversary tandem customers really, really wanted disc brakes on his new tandem. Since the tandem was to be a travel version, the brakes had to be cable actuated. He researched it himself and came up with the new Hope cable-actuated disc brake. He called the company himself and was assured that they had a tandem quality cable-actuated disc brake that was tested and ready for sale as a 
primary brake. At that time, there were no standard disc brake braze-on mounts so we needed the brakes here before we could finish the frame. I called Hope to order the brakes as we began building the frame. I was told that in few weeks we would have the brakes.
Curious about the new brakes, I talked with the representative about them and he told me that he hadn't used them himself, but that they had been ridden extensively on tandems in England. The performance of the brakes was sufficient for use as a primary brake on a tandem, and that he had them on the way to America right now.
Time keeps running like a river
Well, a few weeks went by, and the frame was ready to paint except for the disc brake mounts. A few more weeks went by, and I called Hope again. I was told that there was a delay, but the brakes would arrive soon. A few more weeks went by, and my customer was getting aggravated with me over the delay. I'm not sure that he believed me that I had even ordered the brakes, and it felt like he was about to cancel the whole bike order. I called Hope again and asked "why is there such a long delay?" The representative told me that they were wrong, and there is no Hope cable-actuated disc brake suitable for a tandem. "What!?" I asked in disbelief. He continued "Once we got the brakes here, I put them on my personal tandem. In short, they don't work and I will not sell them for tandem use." I told him of my dilemma with my customer and I asked him to call the customer and let him know the bad news. Much to the credit of the Hope representative, he did that.
I appreciated the fact that this rep. was willing to take responsibility for the error, and help us out by calling the customer. As it went, the customer was disappointed that they would not have disc brakes on their new tandem, but was thrilled that the bike could now be completed. I still see that bike come in for routine maintenance now and then, and I know that the customers are still enjoying it 15 years later.
We did continue to use the Hope cable-actuated disc brake as an auxiliary drag brake through the 1990's and early 2000's. It served as a lighter alternative to the Arai drum brake for those trying to save every ounce on their tandem.