...
SQL> create index idx2 on blahh (id2);

Index created.

SQL> insert into blahh values (1, null);

1 row created.

...

SQL> select 'x' from dual where exists (
  2    select /*+ index_ffs (blahh idx1 idx2) */ 'z' from blahh where
  3    (id1 = 1 and id2 is null) or (id1 is null and id2 = 1))
  4  /

no rows selected