It all depends what you're trying to monitor and what app you are referring to! Are you referring to the Fido app and usage tracking?
If so, if two numbers are on the same account and you have account holder access then you can log in and view the usage for each number, all with the same login and app. Once logged in you'll see a drop-down to choose what number you'd like to see the details of.
I do hope that helps! If you're referring to a different app, please give us more details