When filing separately, you can divide the deductions in any way that is reasonable to both of you. Generally, person-specific deductions like medical expenses, state income tax, and employee expenses should be claimed by the person who incurred or paid them. General marital obligations like mortgage and property tax are often split by who paid which share, but there is no black letter rule. If you want to say that one spouse paid the mortgage and the other spouse bought the food and paid other expenses, so that spouse one claims all the mortgage, that is usually ok. Just be sure that you don't claim more than the total between you, and remember that if one spouse itemizes, both spouses must itemize even if their total is less than the standard deduction would be.
