How to have a unique primary key on two tables

Hi everybody,
I need to have a primary key which has to be unique on two tables.
E.g.:

CREATE TABLE first
( id serial NOT NULL, testo text,
)

CREATE TABLE second
( id serial NOT NULL, testo text,
)

When I insert some text on "first", I would like first.id = second.id
+ 1, and vice versa.
A sort of primary key in common for both tables.

Any hints?
Thank you very much
-- 
Daniel "bodom_lx" Graziotin
- http://daniel.graziotin.net
- http://daniel.graziotin.net/bodom_lx.asc - GPG public key