Foreign Unique Constraint

I was wondering if someone could help point me in the right direction
w.r.t. foreign unique constraints. I'm working on a legacy database
and have a new requirement and am not sure how to do it.

I have something like this

create table table1 (
id SERIAL PRIMARY KEY
extension UNIQUE,
<other fields>
)

create table table2 (
id SERIAL PRIMARY KEY
extension UNIQUE,
<different fields>
)

Basically table 1 and table 2 both have the concept of an extension
that must be unique but the rest of the info in the tables are
different. I need to ensure that if i add an entry to table 1 with
extension 1000 that it will fail if there is already an entry in
table2 with the same extension.

Essentially i need to do something like the following but i get errors
saying this can't be done.

alter table table1 add check (extension <> table2.extension);

It should be noted i am using pg version 7.4.13 and this can't change.

Thanks for any help you can offer.

Jon.