The situation does not seem to be specifically described in either the relevant law or treasury regulations, other than frequent use of the word "individual." Without some other expert to contradict me, I would say the payments need to be 110% of what your tax payments would have been if you were filing single in the previous year (not just half your married tax payments, but equal to what you would have owed if filing single.)
This does not seem like an easy thing for the IRS to calculate, so if you are on the edge, you might want to overpay, or you might have to file an appeal later if they assess a penalty based on your not paying 110% of your married taxes.
