Accessing different databases in a cluster

Is it possible to access tables in one database from another database if
they're in the same cluster? I dont seem to be able to do it; is there
something I have to do or is it impossible?

Ie.
If I have two databases accessible from the same postmaster; one called
db_one and the other called db_two.

%psql -U postgres -p 5555 db_one
db_one=# select * from db_two.mytable;
...

or, from the other perspective;

%psql -U postgres -p 5555 db_two
db_two=# select * from db_one.myothertable;
...

Thanks,
dave