SELECT
    t1.date,
    t1.time,
    COALESCE(t1.windspeed,t2.windspeed) as windspeed,
    COALESCE(t1.winddir,t2.winddir) as winder
FROM t1 INNER JOIN t2 ON t1.date = t2.date AND t1.time = t2.time

CREATE TABLE tblName AS

SELECT
      t1.date,
      t1.time,
      COALESCE(t1.windspeed,t2.windspeed) as windspeed,
      COALESCE(t1.winddir,t2.winddir) as winder
FROM t1 INNER JOIN t2 ON t1.date = t2.date AND t1.time = t2.time