Hi 

Other than the Internal Transfer, one could achieve the same in Inter-Company concept. Though it is little advanced, it can still get wonderful results with separate invoices of each company for recording .

Step 1 - A user of "ABC Sales" Company Login - Creates a Purchase Order (PO ) considering "My company" as Supplier.On Click of Confirm  of Purchase Order  , ABC Sales  will have Incoming Shipments(IN) Created in Draft Mode.

Step 2 -  In turn, in "My company" Log in  -  Sale Order is created with "ABC Sales" as Customer(Automated Sale Order is possible).On click of Confirm of Sales Order , "My Company" will have Delivery Order (DO) created in Draft mode.Process the Delivery Order and deliver it to ABC Sales. By doing this, My company Stocks will be reduced.

Step 3 - ABC Sales Incoming Shipments will be processed (Automated Action is possible) and product will be received  which increases  the stock of "ABC Sales"

Step 4 - For ABC Sales purchase(Step 1), My company is Supplier. So  Supplier invoice has to be generated.
Step 5     -->     For  My Company (Step 2),  logically  ABC Sales is Customer. So  Customer invoice will be  generated.

Same way one can do the reverse scenario and also avoid Step 4 or Step 5 if invoices are not required.

Enable - Inter- Company concept in Settings. General Settings -> Inter Company setting . Post Odoo 8, it is in enterprise edition but one can replicate the code from Odoo 8 in new versions to make it Automated.

PO - Incoming Shipments  in Company 1  will automatically create SO  in company 2 and If Deliver Order (Comp 2 )is processed , Company 1 Incoming shipments will be processed automatically.

Hope this will help in visualizing the scenario.