You can have as many Photopass cards as your heart desires. Get and only use the one in your Mickey Mail, get a new one from each Photopass photographer, it doesn't matter. As of know there is no known limit to the number of Photopass cards you can have associated with a single online account.
However, when you buy Photopass+ you only get 1 Photopass+ card. You can still use normal Photopass cards, Photopass cards from the water parks, the slips from Jedi Training, whatever on your account as well, but you only get 1 Photopass+ card. The only reason you need that special card is to show the CM that you paid for the pictures. That is it. There is nothing else about it that makes the Photopass+ card magical beyond the fact it shows an expiration date. You do need the Photopass+ card to claim the ride or dining photos. The ride photos you can just write down the number and claim it later in the day, while dining you may need to show it at the time of. I don't know if they hold onto the printed pictures all day or not.
Regarding sharing, sharing the Photopass+ is kinda hard unless you are going to be touring in the same parks each day. Just text whomever has the card the ride photo number, and have the other party claim it. Or trade off mid trip, each party gets it for half the trip. Sharing the PhotoCD is real easy, you just need to both add all your cards to the same account online. Do all your edits, and then order the CD once you are done. You would only get a single copy of the cd(s), but you can order a second for $19.95, or make a copy of them yourselves once they arrive. If they only want the CD, and not the Photopass+ features, you could place the order, have them pay have the CD cost ($60) and you pay the remainder of the Photopass+ cost ($109). Then add all the cards to your account, do the edits, and then order the CD using the Photo CD voucher included with your Photopass+ pre order.