OK, an update: It's been at least 1 week since I last checked, but now the sums of the award and tier miles are being displayed correctly in my wife's account.

In fact, when logging in with her account's credentials, our together account shows 19 more award miles than it does when logging in with my own credentials!! (those 19 miles are from 2 small transactions, that occurred in the last 6 days)

Go figure...