All you need to do is this:
Go to the POP box that you want to forward. Click on Custom Filters "EDIT".
When the screen for editing filters comes up, on the first of the five possible choices, select "Simple Filter" from the drop-down menu. Then, in the next box on the same row enter this:
This will keep a copy of the message in the first POP box and forward a copy to the second POP box.