As long as you file a joint tax return, it doesn’t matter how you divide the enrollment for the FSA. As long as your total enrollment is not more than $5000, you will receive the same tax benefit no matter how you divide the enrollment.
However, if you intend to file as married filing separately, the maximum per spouse is $2500, and the spouses cannot use the FSA on their tax return unless they claim that specific child as a dependent on their tax return. So if you have two children and you file separately, your FSA enrollment is limited to $2500 per spouse and each spouse must claim one of the children as a dependent on their separate tax return. (If only one child is enrolled in an eligible daycare program, then only the spouse who claims that child as a dependent could use a $2500 FSA and the other spouse will not qualify to use the FSA at all, if they claim the second child as a dependent.)
FSA enrollment reduces your federal income tax, your state income tax, and your Social Security and Medicare tax. The exact amount that it saves you depends on your tax bracket. For example, in New York State with a married couple filing jointly and a combined income over $110,000, the federal tax rate is 22%, the state tax rate is 7%, and Social Security and Medicare is 7.65%. So the FSA reduces your taxes by a combined total of 36%. If spouse a carries the entire FSA enrollment of $5000, then spouse B would owe spouse A $2500 and spouse A would return 36% of that to spouse B as the tax savings, for a net transfer from spouse B to spouse A of $1600.
Applying the numbers to your exact situation would require knowledge of your total income, and your state and federal tax brackets.
