Normally, a person has to be unmarried in order to qualify for Head of Household (HOH) filing status. However, you can be "considered unmarried" (and therefore eligible for HOH) if you meet the criteria listed in this article: Can a married person claim Head of Household filing status?
It sounds like you both meet the HOH criteria and, if you do, file using the HOH status. Don't be scared off by fears of an audit. If the IRS has any questions, they will send you a letter. Be sure to keep receipts (rent/mortgage, utilities, etc.) just in case.
Also see this TurboTax article: Do I qualify for Head of Household?