you can try this

select acno from rcb_acct
where lgon_pref = 'MYN'
order by
decode(acno ,
'555555666666666','1'
'333333333333333','2',acno) ;

assuming acno is always greater than '1','2' so that default value from decode is the acno and will be ordered after '1' and '2'.

OR

if you want to sort by two groups of union, try this

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