There is no such thing as a 50/50 case.
For tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial parent, is that the child live with you MORE than 50% of the time (sleeps at your home more than half the nights of the year). One of you has to be the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal custody.
In the rare case (could probably only happen in a leap year like 2020), where the time that each parent has the child is exactly equal, then the parent with the higher income (AGI) is the custodial parent, for the purpose of determining who has first priority on claiming the child as a dependent. But then neither parent can claim a Qualifying Child dependent, for some tax benefits because neither parent had the child the required MORE than half the year (no earned income credit, based on that child, and the child would not qualify the parent for Head of Household filing status).
