To make the story short, I'm charting out some holiday possibilities, would like to go CW but don't have enough miles to upgrade both of us.

After searching for mixed class bookings, I know it's possible (though not online) to book WT+ one way and CW the other and upgrade the WT+ leg for half the miles, but is it possible instead to book one person in paid CW and the other in WT+ and MFU that?

The reason I'd rather do it this way is simply TP optimization, and I need us on the same ticket (not just linked PNRs) to be able to select seats for both.