Multiple catalogs per portal are not supported pre-Madrid. Only one Service Catalog can be used with a portal. There are a couple of ways to handle this and achieve the look of 'multiple catalogs.'
Please choose one of these options:
A. Here you are not creating a separate catalog. You will need to create a category under Service Catalog and create new items in the new category. For each new item, define the user criteria in the Available For/Not Available For related lists. User Criteria will control who will be able to view the new items in the category and create requests.
B. Create a separate portal and separate catalog. Add a link to the new portal, if needed. Requests needing approval will appear in the sp portal because the portals share that table.
In Madrid, multiple catalogs per portal are supported. Configure a catalog in Service Portal: https://docs.servicenow.com/bundle/madrid-servicenow-platform/page/build/service-portal/task/associate-portal-catalog.html