I see a familiar sticker on the stoker's seat tube - I could have originally built it
Ibis tandems were so totally cool. They worked incredibly well as either a road or off-road bike. For off-road duty, you can add that second rear brake and run a brake lever that pulls two cables. Magura made a double pull lever years ago. Or like Rumpfy said, connect it to a friction shift lever that the stoker controls bringing back the drag brake.
By converting it back to a mountain tandem, I would assume you mean putting flat bars on it? If so, try using something like the original WTB bars or On-One Midge bars - a drop bar with a lot of flare. These would be a great bar that would be good for using the bike both on and off-road.
The bike looks like it probably was a Cousin It. The Uncle Fester was the base Cousin It frame that was only available in one of two powder coated colors - an aqua or a red. Your bike has several options - like the second rear brake (which doesn't look like it was stoker operated like other drag brakes as I see an unused cable stop on the top tube near the head tube which meant that it may have been connected to a dual cable pull brake lever. The Fester was also available only with canti brake posts, not U/Roller like yours. Yours is also a smaller frame so it didn't have the full-length direct-lateral tube that ran from the head tube to the stoker's bb area.
Year? Is that a 1" or 1 1/8" headset? It kinda looks 1 inchy to me so that, and the decal style would make it something like a 91??? What is the serial number? You can get a better idea of the year based on the decal if you go
here and scope out the decal pages. That site wasn't working for me this morning, though.