In practice, we are splitting mortgage costs, but this takes the form of me paying the mortgage from my personal (not shared) bank account, her paying for other shared expenses, and us settling up the balance between ourselves. Based on our incomes and other deductions, it would be most advantageous for me to claim the entire mortgage interest deduction and her to take the standard deduction, but we're not sure if this is permissible and/or likely to raise any red flags, given that she is listed first on the mortgage and the 1098 was issued with her SSN.
