I bought a house last yer with my girlfriend, we split everything 50/50. does one of us file the 1098 or do we both file it as co-owning the loan since we are not married.

With joint ownership for unmarried individuals, each can only claim the portion of any expenses such as interest or real estate taxes that they pay.

If a Form 1098 is issued and does not include your social security number as the first borrower you need to indicate that in TurboTax. It permits you to state another individual received the Form 1098.

So since both our names are on the loan we would both file as co owners? Or could one of us to file as the owner and the other just not file?

See my response above - if you paid it, you claim it.