If your two TDE files come from different tables with one database,
there might be two soluitions,
1) custom SQL
2) waiting for the future 10.2 version, which might extend union funtion for database
Current union function works for "distinct text files, Excel sheets, or Google Sheets worksheets" according to the link.
Here comes an example using two sheets in one Excel file.
Unioned in Tableau Desktop.
Initially you cannot append the two files with common and unique fields.
As suggested by Lei, first we have to make union of file 1 and file 2, say of 1st Date and from there you can append file 1 and file 2 separately, say of 2nd date.
Hope it works for you.