If you both are going to file married filing separately (versus filing Married filing Joint together), you will need to decide who claims what when dividing up the itemized deductions you have for deductions such as mortgage interest.
Keep in mind that if one of you itemizes deductions, you both have to itemize deductions even if you both do not have itemized deductions greater than the standard deduction.
The mortgage interest will need to be divided based on who actually paid for it. You can only take a deduction for the amount you paid. If you only paid a portion of the mortgage interest, you would include that on your tax return.
The link below explains more about when to file jointly or separately:
Should
You and Your Spouse File Taxes Jointly or Separately?