How to set Picking Rules?

Dear All Experts,

We have 2 Warehouses, like A & B. If a Sales Order is generated for any Item from Warehouse A then if Item is not available during the Pick Release it should be backorder. Here we need to setup a Picking Rule that when Item is not available in Warehouse A then Item should be pulled from Warehouse B and then receive it to A Warehouse . So that pick release can be run sucessful and ship confirm would be taken.

So here I am confused that how to make this type of Picking Rule please any suggestion??

Thanks in Advance...