My 2 cents, if the data sources are on different levels of detail "Data Blending" is your best bet. Have you tried to blend the sources? Calculated fields are data source specific.
Have you thought of using Prep to aggregate the data from the one source to bring it to the same level as the second source - then you could join the 2
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Actually yes I tried that. Ended up not being able to move forward due to the fact I couldn't do an LOD on the secondary datasource. Also there is another part where I had to go into another 3rd level of detail that wouldn't join because my data was grouped at monthly level which would break the 3rd level.