Hack 1: The guy who sold me my tandem didn't have a rear stem. He lost it somehow. There were no stems available with the right diameter clamp size (to the front seatpost), 26.6 mm. I took a 27.2 mm seatpost and cut it and split it. I tied it around the seatpost and ground it down with a bench grinder until I had two segments of shims so the stem would fit on the seatpost.
Hack 2: The bike came with a 3x7 drivetrain. The spaces between the gears were too great. I replaced the rear derailleur, the rear shifter, the chain, the cassette, and the rear hub. I got an 11-speed drivetrain so that required the rear hub. It wasn't clear I could fit an 11-speed cassette on a 10-speed hub, but that worked fine. I reused the spokes and rim to rebuild the rear wheel. The spaces between chainrings on the crankset were too great as a result of my upgrade. The chain tended to fall between the large and middle chainrings. This is a big problem on a tandem because we can't pedal forwards or backwards, and we have to keep the bike upright somehow while coasting to an unplanned stop. I found a used 10-speed crankset and replaced just the right arm and chainrings. I kept the left crank at the stoker's position. Cassettes are wider now, so now our top gear is higher than before, and our low gear is lower than before, and that's very helpful. It's an 11-40 cassette.
