select max value from 2 tables (mysql)
You don't need a source or a date in resultset at all. If so, you may use the simplest

SELECT domain, MAX(score) score
FROM (SELECT domain, score FROM tableA /* WHERE date = @date */
      UNION ALL
      SELECT domain, score FROM tableB /* WHERE date = @date */ ) dummy
GROUP BY domain


or (maybe more optimal)

SELECT domain, MAX(score) score
FROM (     SELECT domain, MAX(score) score 
           FROM tableA 
           /* WHERE date = @date */ 
           GROUP BY domain
      UNION ALL
           SELECT domain, MAX(score)
           FROM tableB 
           /* WHERE date = @date */ 
           GROUP BY domain 
     ) dummy
GROUP BY domain