There should be a common dimension to blend the data source into single worksheet.
For example, when blending Acutal and Target sales data, the two data sources may have a Date field in common. The Date field must be used on the sheet. Then when you switch to the secondary data source in the Data window, Tableau automatically links fields that have the same name. If they dont have the same name, you can define a custom relationship that creates the correct mapping between fields.