Rigid seatpost hasn’t been an issue. Bigger tires with lower pressure create some suspension.
Also, to me there really is no difference between the position of the stoker in relation to the rear wheel than to a single rider to the rear wheel.
So riding a tandem with a rigid post should be no different from a single, with the exception that the stoker has less opportunity to see what’s coming.
In our experience, some communication, loose elbows, and knees, obviates the need for a suspension seatpost. YMMV.
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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.