You must mail each amended return in it's own separate envelope. Since the IRS is still waaaaay behind on processing mailed returns (both normal and amended) due to their shutdown during the pandemic, I would mail them about a week apart. But that's me. A few points to cover here.
1. Print out your returns as originally filed and accepted by the IRS, "before" you start the amending process. When printing, elect to print everything; not just the "forms for your records" or "forms required for filing". Print everything. That way, you get all the worksheets and calculation forms. They'll save your butt big time, if audited, or if the IRS has questions on your amended return.
2. When you mail each amended return, mail it "return receipt required" (not "requested") so you have proof of delivery.
3. When you start your 2022 return, which I assume will be after you amend the prior year's return, make sure you import your data from the "amended" 2021 return, and not the originally e-filed return.
When e-filing your 2022 return, if it gets rejected because of an incorrect AGI, it's probably because your AGI changed when you amended, and the IRS has not yet processed the amended return. In that case, just use the AGI from the originally filed, accepted and processed 2021 tax return. (Hence, one of many possible reasons to print the prior returns before you amend them.)
