The IRS claims that they process amended tax returns in the order received. So if you file the 2020 amended return before the 2021 amended return a letter from the IRS about the 2021 amended return should not be necessary. However, since this is the IRS there is no guarantee they will follow their own procedure.
You should mail the 2020 amended return using a tracking service from the US Post Office so that you know that the IRS actually received the paper return.
For the 2021 amended return you should wait at least one month before filing the return. The 2021 amended return should be able to be e-filed using TurboTax if e-filed on or before October 17, 2022. However, if for some reason the return cannot be e-filed then again use a tracking service for the mailed 2021 amended return.
