試してみてください:
(
select
*
from
_member_facebook
inner join
_member_pts
ON
_member_facebook._fb_owner=_member_pts._username
where
_member_facebook._promote_point = 9
ORDER BY RAND()
limit 2
)
UNION ALL
(
select
*
from
_member_facebook
inner join
_member_pts
ON
_member_facebook._fb_owner=_member_pts._username
where
_member_facebook._promote_point = 8
limit 3
)
ただし、ORDER BY
を入力する必要があると思います 2番目のクエリの最後にある句