Note - check with your state.  For example in Maryland we don't have a "legal separation".  See from mdcourts.gov: "There is no such thing as a “legal separation.” In Maryland if you and your spouse live apart with the intention of ending your marriage, and if you do not have sexual intercourse during that time, that constitutes separation".

Remember, just like custody rules in divorce or orphan's court - the IRS has their own (and usually different) rules.