Multiple public key encryption Sorry for the newby question but would this work: Alice encrypts a message X with John's public key to form Y. Alice then encrypts Y with Bob's public key to form Z. Alice then sends Z to Bob and John. 

Would I be correct in assuming that to decrypt Z,  Bob would have to use his private key on Z to get Y.  After this John would have to decrypt Y with his private key to get X? 

Thanks
No. Alice encrypted X by John's public key to have Y. e.g. Enc(key_John, X)= Y.
So the only work that John is able to do is decrypting Y by his private key to have X. 

Generaly, first Alice here has encrypted X by John's key, then encrypted the result by Bob's key. Therefore both Bob and John have recieved: Enc(key_Bob, Enc(key_John, X))

So to get X again from Z, you must first achieve Y from Z( by decrypting Z via Bob's key), then achieve X from Y ( by decrypting computed Y via John's key). which means:

Dec(key_John, Dec(key_Bob, Enc(key_Bob, Enc(key_John, X)))

Note that each person only can decrypt the message which is encrypted by his key.