First create a Global Field
Then create a relationship from the Global Field to your second portal Table Occurrence
Then create a button in your first portal. The button should Set Field (Your Global Field) to the ID from the first portal.
Since you then have a new relationship from that Global Field to your second portal, that portal will show only records related to that first record that you clicked on.
If you on other hand still want to see all records in the second portal, use Conditional Formatting on the field and see if Global Field is the same as ID from first Portal, then show maybe another background color to show users that this record is related to the other.
It could look something like this
Then you click on one Category
You can then see what you clicked on and that Categories Sub Categories appear in second portal