I have two tables which both include a date field. Currently I have two portals, one for each table (occurrence).
Is it was possible to display the results of both of these in one portal, and sort by the date?
Technically a portal can only display records from one table. If you need to join two tables then you have to do this manually or change the design and use one table instead of two (since you want them in the same portal, then the tables are similar to some degree; maybe this similarity can go into its own table).
Sometimes developers use the so-called virtual table technique: they create a table with, say, a field with the record number and a bunch of calculated fields that pick their values from elsewhere, for example, from prefilled global variables. They create a portal to this table, set up the relationship to display the required number of records, and write the code to fill these variables. This way they can show data that isn't stored in any table, combine tables, etc. But it's an arcane technique, I would recommend it only as the last resort.