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.
