All-UA metal can be split. Why not split it either now if you don't need to be on the same PNR or after you fly the outbound (if you do). Then wait list your wife on the flight she wants if there is no award space. Up to two wait lists are permitted for each segment, so you can wait list two flights on the same day or on different days. If you see the loads shifting, you can change the wait lists.

No fee if you keep the same routing.

If you have other airlines on the PNR, it's hard to split but there are workarounds in some cases.