If your IRS accounts say that each of you received $2,100, that is what you would want to enter for the amount of stimulus received. If you had a second child born in 2021 or that you did not claim in 2020 that would make sense as to why TurboTax determined you should have been paid $5,600 but so far have only received a $4,200 stimulus.
If the IRS has stated on your Letter 6475s that you received $2,100 each and according to your records you have only received $1,100 total, you will want to try to figure out what accounts for the discrepancy. You can check on the IRS website to ensure Letter 6475 has the correct information for your account. If there is a discrepancy between what the website reports and what is on the letter, you should use the amount on the website.
If the IRS shows a payment that you did not receive, you can request a payment trace from them by calling 800-919-9835 or mailing a Form 3911 for the missing payment. Problems are more likely if you closed the bank account that was receiving direct deposits or if you requested checks.
Since the payment trace is unlikely to be resolved quickly, you can still file using the numbers the IRS reported on Letter 6475. This is likely to get your taxes processed the most quickly, but if they later conclude that the number they reported was not what you received, you may have to amend your return in order to get the missing amount.
