Maybe it’s just me but my Monzo Premium card (beautiful though it is) is too similar to my Monzo business card when I use them in my iPhone wallet. Keep using with wrong one. Can you make one more silver and the other more white?
Try storing one of them the other side up. Or load the personal card into Pay?
I think they do mean Apple Pay (“iPhone wallet”)
Easiest solution is to remember which one is set default