Do I add both 6475 impact payment amounts to get our joint filing amount?

If you each received $1.400 for the 3rd stimulus payment and this is reflected in the 6475 letters you received from the IRS then $2,800 is the amount that the TurboTax program is showing you.  Answer Yes that is the amount you received.

If you each received the third EIP payment then your own bank records ought to show deposits of $1400 per person--or one deposit of $2800  --- unless you got more than that by claiming a dependent.

Check your online accounts under each name and SSN on the IRS site:

https://www.irs.gov/payments/your-online-account

This is not something you want to make a mistake on.   If you received the payments and incorrectly say that you did not receive the money, it will put it on line 30 of your 2021 Form 1040.   Then your return will go under review----the IRS will see they sent you the money----they will reduce your refund by the amount on line 30--- and you will wait for a long time for all of that to happen.