Yes, when you split the mortgage with someone who is not on your tax return, then you only enter the portion of the mortgage and property taxes that you paid. You also keep a note with your tax information to explain that in case you are ever asked about it.
In order for your mortgage payments to be eligible for the interest deduction, the loan must be secured by your home, and the proceeds of the loan must have been used to buy, build or improve your main residence, plus one other home you own that you also use for personal purposes.