Two single persons can deduct more mortgage interest than a couple that is married. If you own a home with someone other than a spouse, and you purchased 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 reside 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 purchased your house after December 15, 2017, then the reduced $750,000 mortgage limit from the TCJA applies. In that case, for two single people, the maximum deduction for mortgage interest is based on a ceiling of $1.5 million.