My Warehouse is splitted in 2 locations : under customs authority area (location A1) & customs cleared stock (location A2)

Here is an example: Loc#A1: qty=3 / Loc#A2: qty=4. The customer order quantity is 5.

I need to consider the entire warehouse as available to sale. So I defined a global stock location (location A) as father of these two locations A1 & A2. Everything is OK with the sales order process. No warning during order entry. Product form based on global stock location Loc#A indicates physical qty=7, ordered qty=5, available qty=2. But...

I need to authorize picking only in Loc#A2. In this situation only 4 products can be delivered until I do manualy a internal stock movement from Loc#A1 to Loc#A2

Note : picking in Loc#A1 is legally forbidden and the internal stock movement has to be manual because it implies to follow a specific business process (out of OpenERP).