1. WAIT for the 2021 return to be fully processed and then you can amend federal and state. You will have to remove any 2020 income from that incorrect return.
2. Not sure what you mean by file them "simultaneously" --- your 2020 return has to be filed by mail. It cannot be e-filed. You may be able to e-file your amended federal 2021 return, although it may be necessary to mail file the amended state return.
3. You can seek the recovery rebate credit for the 1st and/or 2nd stimulus payment on a 2020 return. The 1st stimulus payment was for $1200 per taxpayer and $500 for dependent children under the age of 17; the 2nd stimulus was $600 per taxpayer and $600 per dependent child under the age of 17.
You can only seek the 3rd stimulus payment-- the one for $1400 per person-- on a 2021 return as a recovery rebate credit. If you already received the payment you must indicate that you received it when you are asked about it by the software.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
