You can configure your SOR to first assess the primary location (closest to the customer) for complete order fulfillment capability. If the primary location can fulfill the entire order, it processes the order. However, if it cannot, the system should automatically check the secondary location (the other side of the US) for complete fulfillment capability before processing the order there. This ensures that orders are not split between locations, aligning with your requirement.
In cases where the primary location cannot fulfill the entire order, the system should be designed to automatically fall back to the secondary location for fulfillment, as long as the secondary location can fulfill the order in its entirety.
This guide has more information: Shopify Help Center | Order routing