I recently purchased a used tandem.
The stoker's seatpost is 27.2 mm diameter, and I'm not having any trouble with that.
The captain's seatpost is a weird suspension seatpost that is less than 27.2 mm, and is shimmed to fit the seat tube; the stoker stem also has a shim between it and the captain's seatpost, and doesn't get a good grip on it.
I am investigating replacing the captain's seatpost with a conventional one, but it seems that after 20+ years, the seatpost binder has compressed the opening down to 27.0 mm--it will not fit a 27.2-mm seatpost, and my micrometer reads closer to 27.0 than 27.2. It's a brazed-on binder, not a separate collar, and the binder "ears" are extremely close together.
I'd like to loosen this up to fit a 27.2-mm seatpost, but am not sure of the best way to do that. I suppose it is possible that the framebuilder (no longer in business) really did build this for the captain to use a 27.0-mm seatpost, but that seems very unlikely. The frame is steel (853).