You'll log into your account, which you have already created. There is a space where you can register a card. When you have done this, there is an option to register and additional card. This option will always be available so you can add as many cards as you like.
What I did last year was, on our last night I took our cards (we ended up with 2 because I left it behind one day) and asked them at the photo place in Epcot to put all the pics from both cards onto one new card so I only had one.
I also suggest making a note of the number on the back of the card/s, as well as writing your surname and resort name on the back of your cards/s. Not only will you recognise your card when the PP photographer hands it back to you, but if you lose it, they can trace your photos and issue you a new card.
Last year on a very busy day at a meet and greet, our card got switched with someone else's. Had I not stopped to view our pics on the way out of the park I would never have noticed. The other people were viewing their pics and saying that some were not their's and I spotted photos of DD on their screen. I handed over my card and sure enough, the first 100 odd pics were of them .... and they were leaving the next day!

Lesson learnt!