I don’t have experience with 1x12 on a tandem (we are running 2x11 which works well for us.)
I do use 1x11 on my mountain bike, and that experience tells me that I don’t believe I’d be happy with a 1x setup on a road tandem.
On my mountain bike, to get a sufficiently low end, and decent spacing, the top end is a bit limited. I could go to bigger chain ring to up the top end, but that would also require a cassette with a bigger big cog and poorer spacing.
It’s not a big deal on the MTB, where top end speeds are typically much lower. However on a road bike, if you want to be capable of pedaling in the mid 20’s at a reasonable cadence, while also having a relatively low climbing gear, even with 1x12, it’s going to be a challenge to do so with reasonable gear jumps.
Of course all this depends, on how fast you want to go on the top end; how low a climbing gear you need, and how big a jumps are acceptable to you.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.