Ubuntu postgresql 2 databas in two hard disks
PostgreSQL has no problem with the data on 2 hard-disks. The best thing that you can do is create a tablespace on your second hard-disk and move one of the databases to this tablespace.

CREATE TABLESPACE somename LOCATION 'directory-on-new-disk';
ALTER DATABASE name SET TABLESPACE somename;


Also see this post: Postgresql: Move database to new tablespace.

If you use LVM then you can also consider to use this to extend the logical volume on which your database is.