Put the union in a subquery and then use SUM()
to add them together.
SELECT SUM(c) total
FROM (SELECT COUNT(*) c FROM events.history WHERE firstTime > UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 1 HOUR)) AND (severity='5' OR severity='3')
UNION
SELECT COUNT(*) c FROM events.status WHERE firstTime > UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 1 HOUR)) AND (severity='5' OR severity='3')
) u