I think if you plan to use airline branded credit card for lounge acces, you have to be flying that airline to be admitted to their lounge. If you have a credit card which provides you access to a lounge as its own benefit, not a crad that's marketed by an airline, you can access the lounge that has an agreement with the credit card regardless of the airline you fly. I have used Diner's Club lounges, even if they were operated ny an airline when I was flying some other airline.