for
select pn.u1, pn.u2 from pn join cw on pn.u1=cw.u1
where cw.u2 = pn.u2

or

select pn.u1, pn.u2 from pn , cw where pn.u1=cw.u1
and cw.u2 = pn.u2

dont forget to check that you have an index on CW(U1,U2)

An other way, much mo brutal, that doesn't need any index but only
available in FB 2.0, could be:

select u1,u2 from
(select distinct u1,u2 from pn
union all
select distinct u1,u2 from cw)
group by u1,u2
having count(*)>1

;o)
Regards
PAscal