Thanks for reaching out to the Community, @COrtiz.

I have some information to share with you that can help troubleshoot the connection between American Express and QuickBooks Online.

When you try to download transactions, do you still encounter the Error 105? If so, we can perform a manual update to resolve the connection issue. 
Here’s how:

1. Select Banking from the left menu.
2. On the Banking page, click Update.

You may take a look at this article for more information: How to fix banking error 102 and 105.

When it comes to the company file that has two American Express credit cards, you can set up one of them to be the parent account, then the other as a sub-account. This way, you can connect them together in QuickBooks Online. 

You can create a new parent account and add a sub-account by following the steps provided in this article: https://quickbooks.intuit.com/community/Reports-and-accounting/How-to-create-a-new-parent-account-or....

Make sure the two accounts are of the same category type for this process to be successful. You may create up to five levels of accounts, one as the parent with four subs.

For more information about setting up bank or credit card sub-account in QBO, refer to this article: https://quickbooks.intuit.com/community/Banking-and-bank-feeds/About-bank-or-credit-card-sub-account....

That should be it. Please know you can always find me here for all of your QuickBooks needs, I want to ensure your success. Thanks for reaching out and enjoy the rest of the week.