And that might not matter. This should be an easy question and off the top of my head, I would say that they both get to deduct half the interest paid.  It's not as complicated as the situation in this week's Lessons From The Tax Court, but here's that discussion:

https://taxprof.typepad.com/taxprof_blog/2023/02/lesson-from-the-tax-court-mortgage-interest-deducti...