It's a fair question; can I share an account with my partner, but own my own charge card? It works when it comes to shared bank accounts, so why exclude charge cards?
Unfortunately it's not possible to register two charge cards to the same account. This has to do with the fact that the account with all linked information are based upon the unique UID of a charge card. This way all charging sessions and bills are neatly organised.
The UID of each and every charge card is unique and different. So, like we say in the Netherlands: 'unfortunately peanutbutter'; no two cards to one account.