I think you mean the "188" combo unit.
I don't think the smaller-screened 168 has a fishfinder component and anyway it requires you to load charting software to its memory with a cable from your computer's CD reader. Not too convenient, but its a compromise that's made to keep the price down.
The 182 or 182C color unit utilize the Garmin proprietary Bluechart cartidges in a little port down low on the face and this cartographic software is really a thing of beauty to use coming up a channel in the dark or late in the season after the CG pulls the buoys.
I have the 182 monochrome on my boat and though I also have a quality radar, I cannot imagine how I got along without the chartplotter all these years. Its just wonderful.
The 188 is the same unit I have except it has the capability to also function as a fishfinder.
I would question however the utility of splitting such a small screen into two instrument displays, maybe if you toggle from one to the other using the full screen for each, but for my eyes, on my boat, I would have problems with such a small image split in two.
If that's what you want to do, and you do not need a color machine - then consider upgrading to the monochrome 238 - a much larger display, that would better serve you in a split-screen mode, with not all that more footprint to take up on your console/dash.