Join between tables of two or more databases

Hi people. I have 2 databases named as follows:

1) company_a

2) company_b


These two databases do not have the same ER model. However, I want to
access tables on "company_a" there are in "company_b". I want to use
them as local tables (as other databases like Sybase allows, since you
have opened a database link).

The question is: can I do this using remote database servers
(different hosts)? If does not, can I do it at least on local
databases on the same server?