Two single people can deduct more mortgage interest than a married couple. If you own a home with someone other than your spouse, and you bought it on or before December 15, 2017, you individually can deduct mortgage interest on up to $1 million of a qualifying mortgage.
For example, if you and your unmarried partner live together and own the home together, the mortgage ceiling on deductions for the two of you is $2 million. If you get married, the ceiling drops to $1 million. If you bought your house after December 15, 2017, then the reduced $750,000 mortgage limit from the TCJA applies. In that case, your two-person maximum deduction goes down to $1.5 million.