If the person was responsible for the deduction and actually paid an amount, then that's the person who deducts that amount, only the amount he/she paid.
Unless you're dealing with residents of a community property state (only a very few of my clients are) where you're including Form 8958, you won't have a lot of halfsies. If so, follow the instructions for 8958.
Maybe with a joint checking account, both names on deeds, and both spouses agreeing...