The best practice here is to do the join within one dataset (i.e. joining in SQL)
But in cases that you need data from two separate cubes(SSAS) the only way is the following:
- Select the main dataset for the Tablix
Use the lookup function to lookup values from the second dataset like this:
=Lookup(Fields!ProductID.Value, Fields!ID.Value, Fields!Name.Value, "Product")
Note: The granularity of the second dataset must match the first one.