Thanks so much for the replies!
I was intrigued by the IQ Square idea, but I'm not willing to wait an indeterminate amount of time for the meters, and I doubt those will be ready before next year.
Because my wife's bike doesn't have a power meter (I have a Power2max on mine), a "solution" that transfers easily from the tandem to the road bike was attractive, which pretty much leaves pedals (or IQ Square) as the solution.
I did read the DCRainmaker 2017 rundown on power meters. My take was that the Garmin Vector 3/3S was the best of the lot. But Garmin was out, and won't have more for six weeks, at least. And our local bike shop only had the Vector 2, which isn't attractive. So I was thinking about the Powertap P1S, when I decided to see any online site had the Vector 3S. And they did. So I've ordered them, supposed to arrive tomorrow.
After we've had a good chance to use them, I'll report back.