GOP- Sourcing for Back-to-Back items

Summary:

Our Customer has a requirement linked to the sourcing of Back-to-Back items. However, the current design, based on a common core for multple countries, does not give the expected results. Can you please advice how to set up sourcing in GOP for back-to-back items to meet the requirements as described below.

There are two different legal entities, bot with their own warehouses.

Expectation is that if item X is ordered for demand class A it will check stock in warehouse A first, if no stock available it needs to check for stock in warehouse B. Only when no stock is available in any of the warehouses, the item needs to be bought from a predetermined supplier.