From the wiki:
8. Do I have to use my own AAdvantage number on applications?
Yes. The name on the card application must match the name on the AAdvantage account. You cannot apply for multiple cards for different people and use the same AAdvantage account number. AA does not care how many Citi accounts you have so there is no harm in having your AAdvantage account number associated with multiple cards in your name. If you forget to add your AAdvantage account number to your application, an AAdvantage account in your name will be created and can later be merged with your existing account by calling AA.