Legally separated and live in basement, I support myself and my dependent son who is in college.
My legally separated spouse lives in upper half of house and supports her 3 children, 2 in college, 1 in high school.
According to one answer I read on here it said two people can live at the same address and both claim HOH if they both are taking care of their own dependents.
This is sticky water and not been able to find a clear answer as we were a blended family and have our own children.
If you have a permanent legal separation or a divorce decree, you will have no problem with claiming head of household in the same structure as your former spouse.
However, if you are still considered married, making a claim of "living separate from your spouse for the last 6 months of the year" might cause some issues. When the IRS examines tax returns, the agents often take a "facts and circumstances" approach with rules like dependents, filing statuses, and itemized deductions. This means that the IRS will take a rule like living separate from your spouse for the last 6 months of the year, and disallow your use of the head of household filing status.