Are you preparing returns for both former spouses (which may not always be the best idea) and are looking for a way to proforma both returns?  Honestly, it may not be worth the complicated efforts you will need to make because it is a manual process.  PTO does not have a function to split MFJ returns automatically.

1. Make a copy of the original return so that the filing copy will remain intact;
2. In the newly created return, you need to make sure your prior year return had every income, expense, and credit coded corrected for TSJ;
3. Check the MFJ comparison to make sure the allocation is in order;
4. Make one more copy of this return;
5. In the first copy you made in #2, remove all the entries coded S and manually split the amounts coded J;
6. In the second copy you made in #4, remove all the entries coded T, manually split the amounts coded J, and, finally, recode S as T;
7. Proforma both of these returns to create their 2019 returns; and
8. In each of the 2019 returns, you will need to manually allocate the 2018 taxes from the original MFJ return for F.2210 and YOY comparison.

After reading the above, I think you'd get the idea why it's not worth the effort, even though it's doable.