If your 2017 MFJ AGI does not work, the best and fast way to access the 2017 AGI is to order a transcript from the IRS. Click here
Get Transcript
and select "Get Transcript Online "
On the transcript, look under section "Adjustment to Income" and find the amount for Adjusted Gross Income.
Per IRS, you'll both use the same adjusted gross income (AGI) from your originally filed 2017 joint return. To prevent rejection, don't split or allocate the AGI, even if doing so seems more logical to you.