How do my ex-spouse and I apportion 2019 refund to this year's federal tax return as divorced singles if last year was filed as married filing jointly?

The treatment of overpayments on past jointly filed tax returns is similar to the IRS's guidance for estimated tax payments. Sec. 6402(a) provides that overpayments should be credited against the tax liability from the person who gave rise to the overpayment. In other words, an overpayment cannot be credited to the person who did not pay tax to generate the overpayment. If both spouses contributed to the overpayment on a prior jointly filed return, the overpayment needs to be apportioned to each spouse proportionate to his or her tax obligations in the current year. This calculation would be similar to that of the estimated tax payment calculation described above from IRS ­Publication 505. https://www.irs.gov/pub/irs-pdf/p505.pdf 

A problem identified in numerous cases is that the IRS will ignore these calculations and simply allocate the entire overpayment to the first taxpayer (on the previous joint return) without allocating any of the overpayment to the spouse. This can be the case even if a suitable calculation was done and documented on the taxpayer's and spouse's separate returns.  

what a tax pro would likely do is two married filing separate or single returns for 2019 to compute how much of the $3,000 is allocable to each. (some pro software can do this automatically from a joint return). if the irs allocates differently for 2020 they calculate what one should pay the other