The IRS does not follow state custody orders, the IRS goes by wherever the child physically spends the larger number of nights. It’s not possible for a child to spend half a night in each parent’s house, so one parent will have 183 or more nights and the other parent will have 182 or fewer nights.
In the event that the number of nights of custody is exactly equal, then the tiebreaker is whichever parent has the higher adjusted gross income. Exactly equal custody is rare, but it might happen in a leap year, or it might happen if you separated in the middle of the year. (For example, the child might spend 165 nights with both parents before the separation, and then 100 nights with each parent after the separation.)
