If you have two or more Apple Watches, you can go into the Apple Watch app on your phone and tap the “All Watches” link at the very top of the screen. From there, it’ll show all of your watches with a checkmark next to the currently active one. At the top, there’s also a switch to turn on and off “Auto Switch” which lets you choose the active watch manually or let the OS pick for you. The only time I’ve found turning Auto Switch off and doing a manual select to be needed/useful is if I’m trying to force a watchOS update.

Again, I don’t think this is going to help your spouse’s use case.

I know this won’t help OP’s spouse’s issue, but it might help someone else in the future:

I have two watches, a new one I wear during the day and the older one it replaced and I only wear for sleep tracking. I hadn’t been getting the sleep tracking for the past week, just pics like the one she posted for him.

It turned out that the issue was that I had turned off the “Auto Switch” a week ago to force an update through, but had forgotten to turn it back off. As soon as I turned it back to Auto Switch, the watch connected and this week’s accumulated sleep data synced.