It's the "scroll through" part that can be sticky. Simply scrolling the portal won't trip a script trigger and can't update the second portal. But you can click a button in Portal 1 to see related records appear in portal 2. You can add a button specifically for that purpose or select all the fields in portal 1 and use button set up to turn the entire portal row into a button that you can click.
The details for setting this up can be found here: Need layout solution for nested portals...