So I have a Chase MPE and I remember that when I added an AU to my account, I just had to enter the AU's name; no SSN or other details required.
The AU card also has exactly the same card number, expiry and security code as my own (only the name is different). For all intents and purposes, it seems that Chase treats it as 'one account', including the earning of rewards. I suspect that if either myself or the AU were to misplace our MPE card, both cards will have to be re-issued.

Now I also have an Amex Hilton card, and Amex asks for the SSN of the AU, and states that a separate monthly statement will be generated, and from what it appears, a different card with different card number will be issued for the AU. My question then is, how will the rewards earning for the AU card be managed? Will it only accrue to a separate HHonors account belonging to the AU? And I am guessing that the AU's credit report will start showing this Amex Hilton account?

I vaguely remember Chase mentioning (when I was adding AU to the MPE) that the account will show up under the AU's credit report too. But I can't fathom how they'd get that done without the AU's SSN to begin with.