You probably have your answer already since this post is 3 months old... so maybe this will help others out... just press the Pages button twice and your screen "focus" will change to the other side of the split screen. Now your zoom keys will work for that side. To go back to the GPS side, just press Pages button twice again. You should see an orange outline on the side that has "focus".
'05 Ranger 519VX Yamaha 200 HPDI