I really thought this was going to work, but it didn't. I merged the accounts like you said, and then set up the bank feed by logging into Amex. When it asked for me to select my account, the newly merged account was in the list, so I thought all was well! Went and downloaded the transactions. When i opened the file, instead of just giving me the "import transactions now" screen, I got the box asking me to select the account from a drop down menu.

It lists Financial Inst - Amex web connect

Type of account - Credit card

Then the account number

Under that is says "QB does not have an online account to handle these transactions."

Below that is asks me to select the account from a drop down menu. In this particular menu the Amex account was NOT listed. Ugh.