You can either create a separate field after join from query editor and then use DAX .
Follow below steps for :
1. Once you have connected to the mysql database and selected the tables, click on edit.
2. Join the 2 table by common fields and then make the 2nd field in 1st table.
3. Now apply sum over both the fields with DAX code and create a column.
4. Now the sum of both fields is in new field. sum = [col1] + [col2]
Hope this helps you.