Just enter the first one into your return and if you qualify for more it is added to your refund.
The Stimulus Checks are really based on your 2020 return we are filing in 2021. So any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.
There will be a screen with 2 boxes for each round. If you didn't get one enter a 0 in the box.
When you go through the federal review tab it will ask how much you got if any. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus."
After you have entered your tax information and are ready for the Federal Review, it will be the first question.
If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.
The Recovery rebate credit is on 1040 line 30. It either adds to your refund or reduced a tax due.
The first answer is great so far as getting the second stimulus through TurboTax. You can investigate, and request, your second (and any upcoming) stimulus checks at https://www.irs.gov/coronavirus/get-my-payment as well. Click the blue "Get My Payment" button, enter some simple identifying information, and the site can tell you if the check has already been sent, or if not, how to request it be sent.