Verwijder tracking_id uit de subquery's en gebruik union :
(select trv.requested_date, trv.requested_status
from tbl_trackvalue as trv, tbl_tracking as t , tbl_offers as off , tblusers as usr
where t.id=trv.tracking_id and off.id=t.offer_id and usr.id=trv.tr_user_id and usr.id='1454'
and trv.payment_status='pending' and trv.requested_status='declined' group by trv.tr_user_id, trv.requested_date order by trv.requested_date asc )
union
(select mlc.requested_date, mlc.requested_status
from tbl_trackvalue as trv ,tbl_tracking as t , tbl_offers as off , tblusers as usr, tbl_mailchimp_trackvalue as mlc
where trv.tracking_id=mlc.tracking_id and off.id=t.offer_id and usr.id=trv.tr_user_id and usr.id='1454'
and mlc.payment_status='pending' and mlc.requested_status='declined' group by trv.tr_user_id, mlc.requested_date order by mlc.requested_date asc )