They can't, but the maximum limit on both mortgage interest ($100K) and the indebtedness on which you can deduct interest paid ($1.1 million) apply per-taxpayer for unmarried co-owners. You still can only each deduct what you actually pay. This is a result of a 9th Circuit City decision IRS decided not to appeal that almost certainly want the intent of the law, and was not the way the law was applied prior to the decision.
I'm surprised Republicans haven't framed it as a new marriage penalty and jammed a “fix” that would increase the limit for married taxpayers to double the single-taxpayer limit into one of their tax bills, since it would let them champion marriage, rail against the perfidy of the Ninth Circuit, and cut taxes on the rich all at the same time.