If you co-own a home with a partner, you would only deduct the amount of mortgage interest that you actually paid. If you didn't pay any mortgage interest, then your partner would claim 100% of the mortgage interest. If, for example, you paid 56% of the mortgage interest, enter that amount. Your partner will enter the other 44%.
Additional information: How do I deduct mortgage interest if I co-owned the home?
Related information:
okay but waht if it is paid from a joint account?
If you each paid an identical amount for the year, then the amount you would put on your returns would be 50% of the total paid for each of you.
The general rules are:
1) You must be legally obligated to pay it
2) You must have actually paid it.
From the "sound" of your post, I'm guessing this property is at least in a partnership that files the 1065 partnership return. So each claiming 50% on the K-1 shouldn't be an issue at all.
Otherwise, if each of you report it directly on SCH E page 1 with 50% ownership, it still should not be an issue assuming the joint account is for rental income/expenses only.
With a joint account I assume that's where the rental income gets deposited, and where all rental expenses are paid from. Sl each claiming 50% of all income/expenses should not be a problem to deal with, under the rare chance you're ever audited on it.