Fam. Code, § 3426

(a) Except as otherwise provided in Section 3424, a court of this state may not exercise its jurisdiction under this chapter if, at the time of the commencement of the proceeding, a proceeding concerning the custody of the child has been commenced in a court of another state having jurisdiction substantially in conformity with this part, unless the proceeding has been terminated or is stayed by the court of the other state because a court of this state is a more convenient forum under Section 3427.

(b) Except as otherwise provided in Section 3424, a court of this state, before hearing a child custody proceeding, shall examine the court documents and other information supplied by the parties pursuant to Section 3429. If the court determines that a child custody proceeding has been commenced in a court in another state having jurisdiction substantially in accordance with this part, the court of this state shall stay its proceeding and communicate with the court of the other state. If the court of the state having jurisdiction substantially in accordance with this part does not determine that the court of this state is a more appropriate forum, the court of this state shall dismiss the proceeding.

(c) In a proceeding to modify a child custody determination, a court of this state shall determine whether a proceeding to enforce the determination has been commenced in another state. If a proceeding to enforce a child custody determination has been commenced in another state, the court may do any of the following:

(1) Stay the proceeding for modification pending the entry of an order of a court of the other state enforcing, staying, denying, or dismissing the proceeding for enforcement.

(2) Enjoin the parties from continuing with the proceeding for enforcement.

(3) Proceed with the modification under conditions it considers appropriate.