Nevermind, I found the correct answer in the TurboTax help:
"If your Form 1098 "Mortgage Interest Statement" has interest for the months you didn't have a home office, you'll need to enter the total interest amount as if it were on two statements: one for the time period you had the home office, and one for the time period you didn't have the office."
...
"Example 1
You lived in the same home all year, and have a Form 1098 for $6,000, covering all 12 months you paid interest. You began using your home office on August 1, and so have 5 months of interest for the time you used your office.
Here's what you'd do:
1. Click on the Add Another Lender button and enter a record for the interest from August 1 to the end of the year.
2. When you enter the lender's name, you might want to add that this record is for the office months only (for example, "Lender X -- August-December").
3. When you get back to the "Let's Confirm Your Mortgage Interest Applies to This Home Office" screen, you should see a record with the name, the interest amount, and the box checked to indicate that the interest has been accounted for on this office.
To enter the rest of the interest, you have 2 options:
* Repeat the 3 steps above, entering a record for the interest from January 1 to July 31. When entering the lender's name, indicate that this record is not for the months you used the office. When you get back to the "Let's Confirm Your Mortgage Interest Applies to This Home Office" screen and see the record, uncheck the "Yes, include for my home office deduction" box. When you go to the next screen, we'll recalculate your home office deduction, and also add all of this interest to your Schedule A as an itemized deduction.
OR:
* Enter the remaining mortgage interest later, in the Your Home section of Deductions & Credits. All of this interest will be added to your Schedule A."
