Its not a custody agreement that allows you to each claim HOH. The only thing that counts for HOH is which home did the child actually sleep more than half the nights of the year (183 or more nights).
If both children slept with one parent more than half the nights of 2019, then that parent is the only parent who qualifies to file as HOH, even if you agree to allow the non-custodial parent to claim the child tax credit.
Going forward, it is certainly possible for you to arrange physical custody of the children for 2020 and future years, so that one child spends 183 or more nights with parent A and the other child spends 183 or more nights with parent B.
