Hi @steph_allen and congrats on the new house!
A domestic partnership is not a marriage under the laws of any state I'm aware of. If you are not married, it is not allowed to file as married. Having a family HSA plan or being on the same health insurance would not require you to file as married.
If you each paid half the cost of the mortgage and both live in the house, you are exactly right - you split the deductions for interest and taxes and each take half on your return.
Be aware that only ONE person's SSN will be on the Form 1098 from the mortgage company - simply because there's only room for one number in that box. The person whose SSN is not on the form will need to report their share of the interest as "interest not on a Form 1098" because there is not Form 1098 with their name and SSN to match to. Otherwise, they might get a non-matching letter from the IRS asking for proof of the deduction.
Hope this information is helpful!
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