In California, can two separate counties each make
Someone pay for shild support at the same time?
That's a double order for the same amount for the same child.

No, only one court has legal juridiction over the child custody and child support, One court can help inforce the order of the other.

The court without jurisdiction should be given a motion to dismiss for not having jurisdiction over case.

The court where the custodity agreement occurred should be the one issuing the order