According to the IRS, taxpayers who share the same physical address must prove that they live as separate households, and that they have independent lives outside the residence. Some factors that can weigh in favor of two separate households sharing the same physical residence might include the following:

• Each family has separate telephone lines.
• The taxpayers maintain separate finances and separate bank accounts.
• Neither family contributes financial support to the other.
• The adult taxpayers have separate bedrooms.
• The children have separate bedrooms.
• The family members don't celebrate holidays or birthdays together.