TurboTax offers the following advice in the article Can We File Two Primary Residences if Filing a Joint Tax Return?
If you cannot easily determine which residence is your main home, there are a number of factors to consider that will help you identify which one it is. Generally, the residence where you receive mail, the address listed on your tax returns and printed on your drivers’ licenses will identify which residence is your main home. In addition, each home’s proximity to your employer and your spouse’s employer, the place where your cars are registered and the place where other family members reside is also indicative of where your main home is.
