Store Shared by Two Operating Units

I have setup a store with a global blanket shared by two operating units. I want both operating units to be able to order from the store, but when we a create a requisition for the operating unit which the blanket was not created against, we can't create a purchase order against it, it asks us to select a valid value for the blanket. Ideally I would like the PO to be created against the store in the operating unit on the blanket, but I can't even do that even if I switch the operating unit. Does anyone know how