multiple referential integrity

suppose i have two tables whose primary keys i want to be generated from
the same sequence and a third table where i want to establish a foreign
key based on a primary key from either of the two initial tables whose
id's are from the same sequence.

e.g.,

t1    t2
--    --
id    id


   t3
   --
   t_id

where t1.id and t2.id each get their values from, say, t_id_seq, and
t3.t_id references t1.id and references t2.id.

i thought this might be possible in postgres by specifying two references
as constraints in t3, e.g.,

create table t3 (
t_id <type> references t1( id ) references t2( id )

this is valid in postgres. unfortunately, the behavior seems to be that
it expects _both_ tables t1 and t2 to have the same value in order to
insert successfully into t3, e.g.,

insert into t1 ( id ) values( 1 );
insert into t3 ( t_id ) values( 1 );

will cause an error because it can't find "1" in t2.id.

is there any way to have stronger referential integrity in such a
situation than by merely relying on the unique values of a sequence such
as t_id_seq, which is shared by two (or more) tables for generation of
primary key values?

any suggestions/explanations would be much appreciated

-tfo