For Excel and Access datasources, the "Tables" have to be in the same document (.xls, .xlsx, .mdb, etc). In the case of Excel, this means "Tables" are really different worksheets or named ranges in the same Excel workbook. You should be able to join data on these different Excel worksheets together using the Multiple Tables feature in the Data Connection UI.
Does this help, or is your case a bit different?
Peace and All Good!
Michael W Cristiani
Market Intelligence Group, LLC