I am in a similar situation. Wanted to upgrade my parents (X, Y) for their trip, however, I (A) have enough for one, but not the other.
However, my partner (B) would be able to upgrade one of my parents using his credits.

X & Y has one reservation code for both seats.
A & B are both Elite

So if A upgrades X, and B upgrades Y... would a situation occur where either X or Y would be upgraded but not both? (or one is confirmed, while the other is waitlisted?)

Wouldn't want Mom to sit up front while Dad is stuck in the back... or vice-versa.