The IRS will accept any reasonable approach for dividing up itemized deductions that would otherwise go on a joint return.
If both spouses are filing separately (MFS), you have to use the same method of claiming deductions - either standard or itemized.
What is reasonable? A "50-50" approach is always reasonable - since you are still married, many expenses are based on previously shared incomes and joint debts (e.g. mortgages, car payments, medical expenses), even if incurred more by one spouse than the other. You can also allocate the deductions in proportion to your respective incomes, or based strictly on who is paying the expense.
Alternatively, some deductions can be allocated other than as described above. For example, home mortgage interest and property taxes can be allocated according to whoever is living in the house or whoever is making the payments or some other agreed-upon split. Same for charitable contributions where a joint pledge was made, but only one spouse is capable of meeting the obligation.
The only deductions that ought to be split according to a strict income-based formula would be state income tax deductions and employee-related business expenses, when applicable.
Also, there is a special set of rules that can allow someone who is a custodial parent take the HOH when they are married filing separate. <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p501.pdf">https://www.irs.gov/pub/irs-pdf/p501.pdf</a>
Page 8-10: HOH when filing MFS
Page 24-25: Itemized deductions with MFS
