Assuming the two of you are filing together for the first time in 2021 (congratulations by the way!), you'll want to enter the adjusted gross income (AGI) from each spouse's originally filed 2020 return.
Don't combine your 2020 AGIs into a single figure, as this is guaranteed to cause a rejection. We'll ask for both of your 2020 AGI amounts when you get to the AGI input screen.
It doesn't matter if a spouse filed jointly in 2020 with somebody else. Use the AGI—no matter what the filing status was or who earned the income—from the return filed by that person in 2020.
If you and/or your spouse didn’t file last year and you’re in TurboTax, just tell us you didn’t file last year when we ask you. If you’ve already entered a number for your AGI, manually enter 0 instead.