switch from one instance to another

Dear dba`s

recently I have created an additional instance, so that I have two databases sharing the same kernel.
Question 1: How can I switch from database 1 to database 2 as dba and as

            ordinary user?
Question 2: is it possible to run both databases at the same time ?

I'm running oracle version 6.0.33 on sun/690

Thank for answering in advance!

Dani Steiger, Federal Institute of Technology,

              Departement of Geography,
              Zuerich, Switzerland