SELECT CASE WHEN (SELECT MAX(Column1) FROM Table1) IS NOT NULL
            THEN [display the result]
            WHEN (SELECT MAX(Column1) FROM Table2) IS NOT NULL
            THEN [display the result]
            ELSE 'Unknown' END
FROM dual

SELECT COALESCE( (SELECT MAX(Column1) FROM Table1),
                 (SELECT MAX(Column1) FROM Table2),
                 'Unknown'
               )
FROM dual;