Cal. Fam. Code § 5700.204
(a) A tribunal of this state may exercise jurisdiction to establish a support order if the petition or comparable pleading is filed after a pleading is filed in another state or a foreign country only if:
(b) A tribunal of this state may not exercise jurisdiction to establish a support order if the petition or comparable pleading is filed before a petition or comparable pleading is filed in another state or a foreign country if: