How do I write a SQL (or dwSQL) query across these two different datasets?
Have you tried something like this in SAS?

PROC SQL;
 CREATE TABLE TRUMP_FB_TWEETS AS
 SELECT *
 FROM TRUMP_FB
 UNION CORRESPONDING ALL
 SELECT *
 FROM TRUMP_TWEETS;
QUIT;


Source:http://www2.sas.com/proceedings/sugi25/25/cc/25p109.pdf