If you need to do this, the following should work:
DECLARE @table1 TABLE (source VARCHAR(10), criteria VARCHAR(1))
DECLARE @table2 TABLE (source VARCHAR(10), criteria VARCHAR(1))
INSERT INTO @table1 (source, criteria) VALUES ('Table1', '1')
INSERT INTO @table2 (source, criteria) VALUES ('Table2', '1')
SELECT source, criteria FROM @table1 WHERE criteria = '1'
UNION ALL
SELECT source, criteria FROM @table2 WHERE criteria = '1' AND NOT EXISTS (SELECT source, criteria FROM @table1 WHERE criteria='1')
You may want to post more info about your structure and data as there could be changes to the database that would avoid having to do it this way...
Hope this helps 🙂