Union does not guarantee the ordering of the results. You can, however, add in the ordering information and use that:
With s1 as (Select salary as col, row_number() over (order by seqno desc) as seqnum
from employee
where empno = '111628548' and seqno = 4
),
s2 as (Select descofemp as col , row_number() over (order by seqno desc) as seqnum
from empdetail
where empno = '111628548' and seqno = 4
)
select col
from (select * from s1
Union ALL
select * from s2
) t
order by seqnum