create table A (
  id serial primary key,
  ........
);

create table B (
  id int4 primary key references A,
  ........
);

create function on_insert_into_A()
  returns trigger as
'
begin
  if new.id is not null then
   insert into B (id, ...) values (new.id, ...);
  end if;
  return new;
end;
' language plpgsql;

create trigger trg_on_insert_into_A after insert on A for each row
  execute procedure on_insert_into_A();

insert into B (id, ...) values (new.id, ...);

insert into B (id, ...) values (new.id, ...);

CREATE TABLE a
(
  id serial NOT NULL,
  aaa text NOT NULL,
  CONSTRAINT a_pkey PRIMARY KEY (id)
) 
WITH OIDS;

CREATE TABLE b
(
  id int4 NOT NULL,
  CONSTRAINT b_pkey PRIMARY KEY (id),
  CONSTRAINT b_id_fkey FOREIGN KEY (id) REFERENCES a (id)
    ON UPDATE NO ACTION ON DELETE NO ACTION
) 
WITH OIDS;

CREATE OR REPLACE FUNCTION on_insert_into_a()
  RETURNS "trigger" AS
'
begin
  if new.id is not null then
   insert into B (id) values (new.id);
  end if;
  return new;
end;
'
  LANGUAGE 'plpgsql';

CREATE TRIGGER trg_on_insert_into_a
  AFTER INSERT
  ON a
  FOR EACH ROW
  EXECUTE PROCEDURE on_insert_into_a();

insert into a (aaa) values ('a');
insert into a (aaa) values ('b');
insert into a (aaa) values ('c');

select * from a;

------+------
  id  |  aaa
------+------
   1    |  a
   2    |  b
   3    |  c

------+------
  id  |  aaa
------+------
   1    |  a
   3    |  b
   5    |  c

CREATE OR REPLACE RULE rule_on_insert_into_a AS
    ON INSERT TO a DO INSERT INTO b (id) VALUES (currval('a_id_seq'));