fusionando dos consultas SELECT

Probablemente usaría un UNION .

SELECT `date`
   FROM `tasks` WHERE `user_id`=1
UNION
SELECT `t1`.`date`
   FROM `tasks` `t1`
   INNER JOIN `urls` `u1` ON `u1`.`id` = `t1`.`url_id`
   WHERE `u1`.user_id`=1;