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
