Generally, unmarried co-owners of a house can only deduct the part of the mortgage interest and taxes that they themselves actually paid. If you each paid half, or if you funded a joint checking account to pay the expenses based on how much each person earned, then you will have to figure out how much each of you paid. You do not have to claim expenses you paid, but the other co-owner cannot claim the entire expense unless the other co-owner actually paid all of the expense.
