Official IRS answer
https://www.irs.gov/faqs/itemized-deductions-standard-deduction/other-deduction-questions/other-deduction-questions-2
With respect to mortgage interest, apply the home acquisition debt limit and the home equity debt limit to the qualified residence or residences to determine if all of the mortgage interest can be deducted. If you're unmarried and pay mortgage interest on the qualified residence, the limit on home acquisition debt and home equity debt is applied per taxpayer. See Publication 936, Home Mortgage Interest Deduction for the debt limit amounts.
If each taxpayer paid one-half of the mortgage and real estate tax expenses, then each Schedule A should reflect one-half as deductions. Both of you should attach a statement to your Schedules A explaining how you're dividing the mortgage interest and payments of real estate taxes. Your housemate, who didn't receive the Form 1098, must list the mortgage interest he or she paid on Schedule A line 8b, "Home mortgage interest not reported to you on Form 1098."
Please keep your records on your split of the deductions for the mandatory review period (statute of limitations) since the information reported won't match the records submitted to the IRS.