For linking these 2 tables you can join at data source level.
Note : common column should be available in both tables which will be use as a joining condition.
Can you please post some sample data.(Excel File or Tableau Workbook)
Also can you be more specific about end result.
It sounds like you need to either blend/join the two tables on Currency Code and Date. Then, you can create a calculated field which calculates the exchange rates and pulls from both data sources (e.g. [Table1].[Sales] * [Table2].[ExchangeRate]) I'd recommend a join, if you're using Tableau 10.