I suspect you want this:
select rn, max(basetype) as basetype, max(itemid) as itemid
from ((SELECT @rn := @rn + 1 as rn, basetype, NULL as itemid
from table1 cross join
(select @rn := 0) var
where name = 'test'
) union all
(SELECT @rn2 := @rn2 + 1, NULL, itemid
from table2 cross join
(select @rn2 := 0) var
where itemid = '5'
)
) t
group by rn;