Garmin usually publishes the User's Manual for a new product, on-line in PDF format, when it begins shipping. Since the one for the 178C wasn't on their website yet, I wrote their webmaster. He apologized and said that was an oversight, and the 178C manual is now on-line.
It has good info on the side by side split screen, as well as the new mount.
I like to see a product hands-on, especially the display, in a store before purchasing, and like to become intimately familiar with it before the visit, so it is much more productive. On-line manuals let me do this. Perhaps this will help someone else.
I do have one question for those who've already purchased the 178C. Are the pictures in the manual in color, as they were in the 188C manual, or in greyscale as shown in the on-line version of the 178C manual? Just curious.