My ex husband and I had 2 children. On divorce I had both of them, but our oldest is temporarily staying with him. His income is much greater than mine, but do I still have to pay him for one child based on my income, and he pays for the child I have based on his?

That's up to a court. Until a request for modification is presented to a court and ruled on, the current order stands. Whether you will be required to pay support for the child now living with your ex will depend on state law. ANY question on law needs to include your general locale as laws vary by area.

Unless you and he goes back to court to modify custody and child support, he still pays you even if the child is staying with him temp.

But yes, if you both go to court and change custody and support, you would have to pay him child support based on incomes