How to use MySQL UNION in qa_db_query_sub function?

Although it seems that qa_db_query_sub function will be deprecated in Q2A 1.9.0, but still I would like to ask how to use MySQL UNION in qa_db_query_sub function?

This simple SELECT function works well

SELECT postid FROM  `qa_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1

qa_db_read_all_assoc(qa_db_query_sub('SELECT postid FROM  `qa_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1'));

But this compound function doesn't work:

SELECT postid FROM  `qa_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1 UNION ALL SELECT postid FROM  `another_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1

Or even this doesn't work either

(SELECT postid FROM  `qa_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1) UNION ALL (SELECT postid FROM  `another_posts`  WHERE `type`="Q"  ORDER  BY created DESC  LIMIT  1)

qa_posts and another_posts have the same structure in this example.