Mo. Rev. Stat. § 454.1524: Simultaneous proceedings.

(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:

(1) the petition or comparable pleading in this state is filed before the expiration of the time allowed in the other state or the foreign country for filing a responsive pleading challenging the exercise of jurisdiction by the other state or the foreign country;

(2) the contesting party timely challenges the exercise of jurisdiction in the other state or the foreign country; and

(3) if relevant, this state is the home state of the child.

(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:

(1) the petition or comparable pleading in the other state or foreign country is filed before the expiration of the time allowed in this state for filing a responsive pleading challenging the exercise of jurisdiction by this state;

(2) the contesting party timely challenges the exercise of jurisdiction in this state; and

(3) if relevant, the other state or foreign country is the home state of the child.