The loan is associated with one of your tax ID's. This is true regardless of whether you are married. Unless you each get a form from the mortgage company with half on it (which doesn't fit your situation, since you are jointly liable for the whole mortgage), you will generate an error if you report interest under the wrong tax number.
Here is the workaround:
1) The person whose tax ID is on the form reports the whole amount as on the form.
2) Then this person makes an adjusting entry, showing that they are not claiming half of the interest, subtracting that out (but not associating it with any form)
3) Now the second person can report that interest on their return, again without associating it with any form--because individuals don't have to give each other forms and the mortgage company fulfilled its duty by sending the form to the person whose taxID you both agreed to give the company when you took the loan.
4) Interestingly you note a "share" in an outstanding mortgage principal. You don't have shares if you both are fully liable for the whole loan. So if you need to record the principal somewhere, you should show the whole amount, as that is what you are personally liable for and paying toward.
I hope this helps!
