I would say if the couple isn't at least living together no one would expect you to send out cards signed together.
These days ....
Most people date more people than they will become exclusive with and most people become exclusive with more people than they will live with, and most people live with more people than they'll marry.
If I had sent out holiday cards with the name of every girlfriend I "happened to be with at the time" it would have been a joke!
At the very least everyone you send a card with both names should have met and spent some quality time with you as (a couple).