Hi Michael,
The routing rules are defined at the product level, while you expecting the system to automatically choose a routing for a product based on the presence (or absence) of another product (or category of product) on the same SO.

While this can be set on an SO line while manually creating a SO, but in your case the Orders are automatically created from the website app. You probably require this validation and route assigned prior to order confirmation.

Not-withstanding, on the inventory and routings there are a couple of ways to achieve delivery

1. Create virtual warehouse - This would ensure all delivery orders are defaulted to the virtual warehouse.   https://www.odoo.com/documentation/14.0/applications/inventory_and_mrp/inventory/routes/concepts/stock_warehouses.html

1.1 A delivery order could be assigned to either warehouse manually or even to both by duplicating or creating a back-order. (1 step delivery)
Or
1.2 Have a 2 step delivery so items are picked from individual warehouses and always shipped to customer from main warehouse (2 step delivery)

2. Deliver directly from warehouse to customer: Define 2 delivery operations apply on product category, this would create 2 distinct operations and each warehouse can ship their products directly to customer.

These aren't solution by themselves, but some concepts which could be relevant and applied while designing your solution.

Best regards