Mapping two table primary keys together

I have 2 table with the same primary key.  If I try and do a join on them,
it is very slow unless I explicitly specify in my select statement that
those two keys are equal.  Is there any way to tell postgresql internally
that those two primary keys are equal?  Basically so that in my select
statement I don't have to always specify those keys as equal.

Thanx..

Mike