To automatically split orders with out-of-stock items, you will need to create a Routing Rule with a specific order separation strategy. 

For more information about rules and order strategies, see our Rules and Order Separation Strategies articles.

1.   Click the drop-down under Customer, and then click Rules.

2.   Under the Routing tab, click Create Routing Rule. 

3.   Add a name for your rule in the Description field on the New Routing Rule page.

4.   Next, set Select Orders matching these criteria... to Fulfillable and Fulfillable ONLY in multiple warehouses.

5.   Set the Action drop-down to Separate and select fulfillable as the Separation Strategy.  Your rule settings should match below.

6.   After you've confirmed that above settings are correct, click Save Rule.  Your new rule will display on the Routing Rules page.

Note: It is important to remember that this rule fires on order creation or when an order item is added or removed. If you currently have an order stuck that cannot be filled due to stock being in separate facilities, you will need to manually modify the order.

Related questions:

How do I split an order with out-of-stock items?

How do I create a routing rule to split orders fulfillable at multiple warehouses?