Well, you can make it work in different ways and creating two inventories is one of them.
For example, another way to separate the available stock and the stock in warehouse is to use the Out of Stock Threshold feature. When the product reaches the out of stock value, the product will be shown as out of stock. Imagine that you have 10 items in warehouse, and the out of Stock Threshold value you have set is 8. In this way, when 2 of this items will be sold, the product will be displayed. as out of stock.
The way I recommend to do it with our premium add-on ATUM Multi-Inventory, is to create two inventories and use one with the available stock, and use the other one with the stock in warehouse. If you want that the second one not to be displayed on the frontend (you want just to be informative for you) don't put a price, for example.
To see the info in the report, in case you want to do it with Multi-Inventories, you can Export Data from Stock Central (right top corner), but it will not be displayed in two columns.
In any case, I recommend you to read the documentation of the add-on where you will find useful info about how it works: https://forum.stockmanagementlabs.com/t/multi-inventory-documentation
And the link to the demo site in case you want to try the free trial: https://www.stockmanagementlabs.com/addons/atum-multi-inventory/
Hope this helps,