Александр Боровиков

Используйте UNION:
SELECT t1.id, t1.date, t1.name, 'wall' AS `type` FROM `tbl1` AS `t1`
UNION ALL
SELECT t2.id, t2.date, t2.event AS `name`, 'events' AS `type` FROM `tbl2` AS `t2`
ORDER BY `date` DESC
LIMIT 20