This is not dissimilar to the Network Railcard ambiguity - when purchasing a ticket for someone else with it, do you need a Railcard discounted ticket yourself, or could you use a season ticket or similar?
It could easily be solved by a modification to the T&Cs in both cases, from:
Both named cardholders must be travelling together to enable discounted tickets to be used.
to either of the following, depending on whether this scenario and similar ones are to be allowed or not:
"Both named cardholders must be travelling together using [identical?] discounted tickets purchased using the same Railcard to enable discounted tickets to be used."
or
"Both named cardholders must be travelling together to enable discounted tickets to be used. The tickets need not have been purchased using the same Railcard, e.g. one may be travelling on a season ticket."
This is actually explicit in the Gold Card T&Cs as:
7) If the journey You are making is covered by Your Annual Season ticket You do not need to buy
yourself a ticket. However, if You are purchasing discounted Off-Peak Day Travelcards for
accompanying adults or children and You intend to travel in a London Fare Zone not covered by Your
Annual Season ticket, You must also purchase a discounted Off-Peak Day Travelcard Zones 1-6 for
yourself.
It is very notable how specific this is.