Choose the SKU that you want to use as your inventoried product. Take the other SKU that you would like to have pull from the same inventory and convert that product to a Kit.
Be sure that auto sync is turned off for the product that you are converting to a kit as the quantity on hand for the kit is determined by the component.
No component product would have been added at this point and the quantity on hand balance would be 0.
Add the first product SKU as a component product inside the kit. Set the quantity that the kit includes to your desired value and click save. You will see the quantity on hand for the kit calculated by the component products.