Kore, I've been using a Furuno FCV582 color finder which is a little out of the Garmin league, but as with the Garmin it has both 50/200 khz frequencies and I "always" use the split screen feature. I cannot tell you how many times fish and or bait as well as thermoclines will show up on 1 frequency but not the other. Alot of guys like the wide angle view as it covers more water and "usually" shows more fish, what they are missing though is the fish and bait that is on or near the bottom that will show up alot better on the narrow beam.
Another mistake I commonly see is guys using auto features on there finders, learn how to adjust the settings and you can dial a machine in much better then the auto settings can. Hope this helps a little.