No, you cannot share an AP. There are no pictures on the APs, now, and they scan your fingers with each entry, so if your fingers don't match, you are asked for a picture ID. Sorry, but thems the rules!
As for why APs aren't made from the same material as the room keys. The ticketing system in the parks is only set up to print the "paper" type tickets, it will not accomodate the hard plastic.
If you need a new Ap, stop by Guest Relations while in the park and they will issue you a new one, free of charge. Another perk of that "paper" type, they can be reissued without charge to the guest.