Combine 2 SQL Queries
Define "combine".  My guess is that you simply want to OR the predicates together

SELECT a, b, c
  FROM my_table 
 WHERE (dt_column &lt; sysdate+1 and c IS NULL)
    OR (dt_column &lt; sysdate and b IS NULL)
 ORDER BY id


You may, however, want to do something else.  Perhaps you want to UNION ALL the two result sets, for example, it order to get duplicate rows.