From Casio's SUPPORT page:
How do I use the Duplex LCD feature on my Pathfinder/Sea Pathfinder watch?
Duplex LCD is not a function which users' operation can change its display color. Duplex LCD is just offering various display of LCD with having 2 different LCD inside the watch. The second is used for 'pointer' operations.
http://pathfinder.casio.com/support/