Finding FOREIGN KEY constraints via information_schema

I'm trying to find out which columns of which tables reference which
columns of which tables by querying the information_schema.  I found
the referencing columns in key_column_usage and the referenced columns
in constraint_column_usage - fine so far.

Now consider the following:

  CREATE TABLE t1 (
    id1 INT NOT NULL,
    id2 INT NOT NULL,
    PRIMARY KEY (id1, id2)
  );

  CREATE TABLE t2 (
    id1 INT NOT NULL,
    id2 INT NOT NULL,
    CONSTRAINT t2_id_fk FOREIGN KEY (id1, id2) REFERENCES t1 (id1, id2)
  );

PostgreSQL groks that, and pg_dump correctly generates

  ALTER TABLE ONLY t2
  ADD CONSTRAINT t2_id_fk FOREIGN KEY (id1, id2) REFERENCES t1(id1, id2);

My problem is that, while key_column_usage knows the ordinal_position,
constraint_column_usage doesn't.  How can I find out that it's really
  REFERENCES t1(id1, id2)
and not
  REFERENCES t1(id2, id1)
instead?