Married couple who previously filed jointly and now want to file separately while still married

[rtaxc]

My parents filed their 2014 return as MFJ and applied their refund to the 2015 taxes and they also jointly sent a form 4868 extension form for 2015 with a check for expected taxes.  They have until 10/15/2019 to file their 2015 tax return because of the extension.  I am helping with their taxes and they want to file as MFS.  My question is, do they split the applied refund and tax paid with the extension evenly between the two returns or is all of taxes paid for 2015 reported on one return?

[DoninGA]

A tax return cannot be amended after the due date of the tax return to change the filing status from Married Filing Jointly to Married Filing Separately.

See IRS Publication 501 Dependents, Standard Deduction, and Filing Information page 8 - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=8

Once you file a joint return, you can't choose to file separate returns for that year after the due date of the return.

[dmertz]

A 2015 Form 4868 was required to have been sent by April 15, 2016 and would have extended the 2015 filing deadline to October 15, 2016.  A 2015 Form 4868 filed in 2019 does not extend the filing deadline to 2019 to avoid late-filing penalties.  I imagine that any payment sent with that form would be applied to any outstanding balance(s) ultimately determined when they do file their 2015 tax return(s).

I think the question is asking how to report on separate 2015 tax returns (with no 2015 tax return yet filed) estimated tax payments made jointly.  A payment made jointly (both names and SSNs on the Form 4868 or Form 1040-ES) can be split onto separate tax returns of the spouses in any way they choose:

https://www.irs.gov/publications/p505#SeparateReturnsEstimatedTaxSeparate-0C11868D