Some programs will not allow multiple co-branded credit cards to be linked to the same awards account. For example, I cannot have both USA and UK-issued BA affinity credit cards linked to the same BAEC account. However, I cannot think of another program which rejects transfers from American Express, Diners Club, or Starwood Preferred Guest due to holding one of the program's own cards.
I don't see anything in Emirates published rules which stipulates that only one type of credit card partner can be linked to a Skywards account, but they do have a general clause which would allow them to do whatever they decide.
Where a Partner offer provides Members with the opportunity to transfer or convert points earned with a Partner to Skywards Miles (“Tipping Offer”), any such transfer or conversion shall only be valid if transferred from an account held with that Partner in the Member’s own name and transferred to that Member’s Emirates Skywards Account in the same name. Emirates Skywards reserves the right in its sole discretion not to credit a Member’s Account under this clause 14.4 or to cancel Miles already credited to a Member’s Account under a Tipping Offer, where Emirates has reason to believe that the identity of the Member does not match the identity of the transferor or where there has been any fraudulent activity connected with the transfer or conversion in relation to the Member’s Account.
Only one Emirates Skywards membership account number can be assigned for the transfer of Membership RewardsŪ points. Any subsequent change to the nominated Emirates Skywards membership account number is not allowed.
I think your best strategy is to ask for help from American Express. I would NOT refer to your MBNA card as an Amex card, but rather as a Skywards card issued by MBNA.