If you're Married Filing Separately, you'll both use the same adjusted gross income (AGI) from your originally filed 2016 joint return.
To prevent a rejection, don't split or allocate the AGI, even if doing so seems more logical to you.
If you are still legally married as of 12/31, you can either file Married Filing Jointly or Married Filing Separately. However, since Married Filing Separately is the least beneficial of all the five filing statuses, here's some extra info that may help:
If you were still legally married as of December 31, you are considered unmarried (and therefore eligible for Head of Household and all of its benefits) if all 5 of these conditions apply:
1. You won't be filing jointly with your spouse; and
2. Your spouse didn't live in your home after June (temporary absences due to illness, school, vacation, business, or military service don't count); and
3. Your home was your child's, stepchild's, or foster child's main home for more than half the year; and
4. You paid more than half the costs of keeping up your home during the tax year; and
5. You meet the qualifications to claim the child as your dependent, even if the other (noncustodial) parent is actually claiming the child as a dependent on their return.
You can also be "considered unmarried" for Head of Household if your spouse was a nonresident alien at any time during the tax year and you're not treating them as a resident alien.
Married filers who are "considered unmarried" cannot file with Head of Household status if their dependent is somebody other than a child, for example, a parent.
