What year is "last year"? If you are amending your 2020 tax return, TurboTax will not let you e-file the amended return in any case. A tax professional might be able to prepare an amended 2020 return and e-file it for you.
When you amend from separate returns to MFJ, you only amend one of the returns. You add the other spouse's information to the amended return. You are not amending both returns. For example, you could amend your MFS return, and add your spouse's information. Your spouse's MFS return just gets canceled.
If you amend your own return from MFS to MFJ you are amending a return that was e-filed, so if you are amending 2021 you should be able to e-file the amended return. I don't think it matters that your spouse's MFS return was paper filed.
HOWEVER, until recently the IRS did not allow an amended return to be e-filed if the filing status was changed. It's less than two months since the IRS removed that restriction. I don't know if TurboTax has been updated to allow e-filing an amended return with a change in filing status. Again, if TurboTax doesn't allow it, a tax professional might be able to prepare an amended return for you and e-file it.
