Easiest way to add Location in Inventory App

I have a requirement of a similar Inventory app , however I need the user to be able to select the Location first and then go to Items and Inventory Log. When user opens the app, the app will ask to choose a location as different locations have different stock/target quantities for the same Item. After going through different posts, I realize I need to use USERSETTINGS to allow user to set the Location. However, I am unable to figure out how my key columns should be configured given that item ID is no more unique. In my case, a combination of Item ID and Location is unique. How do I go about setting my data structure? 

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