My return was picked for the IRS national research program, where they audit all parts of a tax return to find out what taxpayers are doing in particular. The IRS agent asked specifically how I calculated the mortgage / property taxes between owners, and accepted my explanation of ownership percentage even though it changed over time.
It seemed like two things were important: I had a method to calculate ownership, and nobody took more than the total deductions (of mortgage and property tax). If you "pick" 0% for you or her, that seems strange. Why aren't you using ownership percentage of the house?
The 1098 will have a taxpayer on it - let's say it's you, and say you own 50%. You divide the amount on 1098 by 2 and put that on your tax return. Your fiance lists the other half. She received it "as a nominee" or something similar, and will list your social security number in that spot on her return (so the IRS can track down the original form, and double check it adds up). Your situation may differ - this is just one possible way it could work. Definitely look up "nominee", and search your tax software for that term.