Using openpyxl, I'm able to read 2 numbers on a sheet, and also able to read their sum by loading the sheet with data_only=True. 
However, when I alter the 2 numbers using openpyxl and then try to read the answer using data_only=True, it returns no output. How do I do this?
You can have either the value or the formula in openpyxl. It is precisely to avoid the confusion that this kind of edit could introduce that the library works like this. To evaluate the changed formulae you'll need to load the file in an app like MS Excel or LibreOffice that can evaluate the formulae and store the results.
openpyxl how to read formula result after editing input data on the sheet? data_only=True gives me a "None" result