How to work with multiple Postgres databases in psycopg2
Yes, it is possible to work with multiple databases at the same time but you're looking in the wrong place. psycopg2 is just a library that simplifies accessing and manipulating data coming out of PostgreSQL but it doesn't go far beyond what you can do with psql. What you're looking to do you can solve on the database level by using Foreign Data Wrappers.

This does become more complicated in your schema definition but brings remote tables from host some.other.server database remote_db to appear as though they live on localhost in database local_db. A simple example on how to hook up the wrappers:

CREATE EXTENSION postgres_fdw;
CREATE SERVER some_remote_server 
  FOREIGN DATA WRAPPER postgres_fdw 
  OPTIONS (host 'some.remote.server', port '5432', dbname 'remote_db');
CREATE USER MAPPING FOR local_user 
  SERVER some_remote_server 
  OPTIONS (user 'remote_user', password 'remote_user_password');
CREATE FOREIGN TABLE local_table_name (id int, value int) 
  SERVER some_remote_server 
  OPTIONS ( schema_name 'remote_schema_name', table_name 'remote_table_name');


Now locally you can just run 

SELECT * from local_table_name


and the query will get executed against the remote host. Needless to say, this requires connectivity between the two servers.

Similarly, if you really have to, you can create a "remote" server against the localhost and point to a different local database for cross database queries. Feels dirty but it's possible.

As @a_horse_with_no_name mentioned though, this is not very efficient. If you find yourself doing this too frequently, you're not getting the most optimal performance and you'd better have very good reasons for keeping your databases separate at that point.