How to do a Join with optionally matching columns
SELECT c1.acreage Maize, 
       c2.acreage Paddy, 
       COALESCE(c1."date",c2."date") "date", 
       COALESCE(c1.region, c2.region) region 
FROM Maize c1 
FULL OUTER JOIN Paddy c2 
    ON c1."date"=c2."date" AND c1.region=c2.region 
ORDER BY "date";


fiddle