Hi Nati
As per standard SAP, storage location is not determined automatically in sales order. Hence the sales order cannot "see" how much stock is present in which storage location. If Availability check ATP for sales order is also at Plant level, it will confirm the full quantity and set the shipping point taken from default shipping point customization.
In your system, does the storage location appear automatically?
If no, you can modify sales user exit through ABAP and make storage location appear in sales order.
During order entry, if partial qty is available in default storage location, there will be 2 confirmed schedule lines, 1. as per available stock and 2. as per planned delivery time. This will be a signal for order entry person so he can add a new line with remaining qty and change the shipping point. This will also auto populate the other storage location which has remaining stock.
In the end, there will be 2 lines with different shipping points and storage locations. During VL10, it will automatically create 2 outbound deliveries as per std SAP.
Hope this helps.
/Dinesh