try this

select acno , 1 as ord
from rcb_acct
where acno in ('555555666666666','333333333333333')
and lgon_pref = 'MYN'
order by acno , ord
union
select acno, 2 as ord
from rcb_acct
where lgon_pref = 'MYN'
order by acno , ord
/