See the limitation of blending.
Just consolidate the source files in a excel workbook in different sheets then connect the consolidated file with legacy connection. Then you can use custom sql to join your both the source. Follow the links for more details
Thanks for your comment.
Unfortunately, I can't combine 2 tables into one. The workbook I attached is just sample data (~1%).The original table size is huge (in database) and they have totally different values for same field/column name. The performance will be drag down if I join 2 tables in query.
Hi Soo Yin Tong,
Mahfooj is right about consolidating
your sheets into a single Excel file.
With Tableau 9.3 you could even use
a simple drag-n-drop UNION on sheets.
Please find the attached version 9.3
of your workbook as an example of the latter.
Hope it could help.
PS There are no values in your EC8 & EC9 columns.