The other day, a colleague asked me an interesting question about mortgage interest deductions. “If two people living together are filing taxes separately, and one has a significantly higher salary, which one should claim the mortgage interest deduction? The one with the higher salary or the other one?” 
They essentially wanted to maximize the possible tax return benefit for one of them.

At first I thought that might something one could run scenarios on and let the numbers dictate the choice. It turns out that the law is actually clear enough on this point that you don’t have to.

From Five Traps To Avoid When Deducting Mortgage Interest

As a general rule, you must satisfy certain requirements –which can best be phrased as questions — before deducting mortgage interest. The requirements that applied to my colleague’s situation are as follows:

• Who paid the mortgage? – only the taxpayer who actually pays the mortgage is entitled to a deduction.
• Who is listed as the borrower? – taxpayers can only deduct interest on debt for which they are legally obligated.

Scenario 3: You and your roommate own a house together, pay the mortgage out of joint or separate bank accounts

Where two or more persons are jointly and severally liable for a debt, each is primarily liable for that debt, and each is entitled to a deduction for the interest on that debt that he or she pays.

• Funds paid from a joint account with two equal owners are presumed to be paid equally by each owner.
• If the mortgage interest is paid from separate (rather than joint) funds, each taxpayer may claim the mortgage interest deduction paid from each one’s separate funds.