You either need to join the two datasets together on the data connection screen:
Or you can use a blend to relate the two datasets together:
I'd recommend using a join if possible, but either should work for your case. Once you have the data joined or blended, you should be able to write your calculation.
you can blend the data, open the first file, then add the second file as a new data source. then you can choose variables from both tables and make calculations.