Good luck trying to split hair with this.  The code is explicit that the couple must not live together at all times during the tax year.

469(i)(5)(B)Taxpayers not living apart.—This subsection shall not apply to a taxpayer who—
469(i)(5)(B)(i) is a married individual filing a separate return for any taxable year, and
469(i)(5)(B)(ii) does not live apart from his spouse at all times during such taxable year.