Hi LMa
You can achieve this with ATUM Multi-Inventory add-on.
Create a product with the two inventories. And in 'ATUM Inventory' tab, select 'Show out of Stock' in Inventory Iteration.
With these settings, once the Main Inventory (online stock) is out of stock, the product will be displayed out of stock for the client.
You can set this setting for an individual product, or in general from ATUM settings > Multi-Inventory.
Best Regards,
Andrea.