SQL> ed
Wrote file afiedt.buf

  1  with t (typ,val) as (
  2  select 'a1',       10 from dual union all
  3  select 'a1',       11 from dual union all
  4  select 'a1',       12 from dual union all
  5  select 'a2',       10 from dual union all
  6  select 'a2',       15 from dual union all
  7  select 'a2',       15 from dual union all
  8  select 'a2',       20 from dual)
  9  select * from
 10  (select t.*,row_number() over (order by val) rn from t where typ = 'a1') t1
 11  full outer join
 12  (select t.*,row_number() over (order by val) rn from t where typ = 'a2') t2
 13* on (t1.rn=t2.rn)
SQL> /

TY        VAL         RN TY        VAL         RN
-- ---------- ---------- -- ---------- ----------
a1         10          1 a2         10          1
a1         11          2 a2         15          2
a1         12          3 a2         15          3
                         a2         20          4