Was he entering his alternate card in MDE in the middle of the 7 AM rush?I am mostly confused about why it wouldn't take his other credit card. And why my son's account wouldn't take my other credit - -3 different non-related accounts.
It is always a good idea to have the CC already on the account before the 7 AM window. Even though you enter it on the MDE app it still needs to be communicated the CC information ( # and Expiration Code) to the Disney back-end system. This is not best done at the 7 AM peak rush.
For your son: was this the same card that you were using on your MDE account? And again was it setup in advance of 7 AM on purchase day?
These issues have nothing to do with Multiple MDE accounts but the CC handling method.
Dave