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?