I asked this same question not long ago too... ( it's our first time at
MNSSHP).
I was told that you can use your same Photopass card. If you get the card at MNSSHP, you get a discount on the pictures for that night. ( so all the Halloween party pics get a discount).
If you are doing the Photopass CD, you can register your "regular" card AND your MNSSHP photopass card and put ALL the pics on your CD.
We're doing the CD so I'm not sure I'm going to order prints through Photopass directly, but we're going to get the special MNSSHP photopass card just in case. Knowing I can compile all my pics from all my cards onto one CD makes me feel better about getting more than one.
Another person also suggested that you take a picture of your Photopass card w/ the number just in case you lose the card. then you can still retrieve the pics when you get home. I thought that was terrific advice.
You can read alot of great tips here....
http://www.stitchkingdom.com/disneysphotopass/ this website helped me ALOT.
You can still pre-order the Photopass CD...here's the link for that.
http://www.disneyphotopass.com/previsitoffer.aspx it gives you a great discount....
Have a great trip !!!