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This is Tableau 10.2 beta feature ..and is not yet available until tableau 10.1.
See below the Join using calculated field in tableau 10.2
(Until then you can use custom SQL to join both table and apply calculation on join column)
You can now use what ever calculation you want on fields that you want to join on left and right side table
I thought I had heard it was an upcoming feature! Thanks for the info.