Split screen and Flex Mode are two different beasts. The first lets you use two apps at the same time (and while the default is to split your screen in half, you can re-size the two 'windows' at will. Flex Mode, on the other hand, for apps that do support it, it's just the SAME app, just two different, independent sections (i.e. watch a video on the upper half while still browsing videos/comments on the bottom or seeing a picture on the upper half while scrolling through your other media files on the bottom).
Personally I find split screen much more useful even on the Flip. I tend to have my messaging app on the bottom and a browser/work stuff on top most of the time. It also helps that you can turn apps into bubbles (floating windows) so I can have two work apps at the same time and just have my messaging apps handy on the bubble for a quick look/reply.