If you want to create a new db instance, the run PSI and give a different set of data files. If you want to create another user and schema, then run create_user.sql.
I assume you are not running two prod. installs. In any case, creating another database instance using PSI gives you the ability to manage/dedicate resources for each of the instances.
We haven't come across any issue with Windchill sharing same instances, but again these are development server with a very less usage.