What matters is your status on Dec 31 of the tax year. If you are still legally married and living in the same household, then you can file either MFS or MFJ. But being separated or "considered separated" doesn't allow you to file anything but MFS or MFJ if there are no kids involved. Overall though, if you file separate returns you will automatically disqualify yourselves for any tax credits you may otherwise qualify for if filing joint. Additionally when filing MFS, if one of you takes the standard deduction, then you must both take the standard deduction - even if the itemized deductions of one is higher.
Likewise, if one of you itemizes then you must both itemize even if the itemized deductions of one of you is zero.
If kids are involved, then it's possible for the one with the kids to file as HOH if certain stringent criteria are met. Now I'm not getting into all the possible scenarios here because only one scenario would apply to your specific situation.
