I finally solved this today. I had the LBS order me a tandem stem. It's a nice one with adjustable reach. But it had a 29.2mm inner diameter where it clamps the front seatpost, and the seatpost is 26.6. The LBS made a shim for me by splitting a seatpost and cutting to the length of the clamping area of the stem. That was too large in diameter, so I clamped it to the seatpost and turned it against my bench grinder. I never milled anything down to a smaller diameter before, but I got it working.

Now my next challenge is to get my wife to try it. She seems skeptical that she'll like tandeming.