The problem I see in applying a split fin to freediving comes from the properties that most folks require out of a freediving fin.
If you try the split fin in a pool or trial setting (like the fin line up over at rodales) they do show an efficiency increase in air consumption. I think that this is mainly due to the fact that there is less actual strain on on the leg when pushing through the water. So for steady swimming through calm water (like the tests) and an unlimited air supply (scuba) it's great. If you look at how someone swims with them they use a shorter, faster stroke similar to using an uphill gear on a bicycle. They are pedaling faster with less effort and when breathing regularly use less air.
Jon has brought up the shortcoming of the fins with his first hand knowledge via u/w hockey. Split fins do not generate much much "umph" from a standstill (or in general, like in a current) and this also results in difficulty in changing directions. All things required for u/w hockey.
When you look at freediving, most people like a stiff fin. I think this is because most freediving is done by short bursts of finning for power/locomotion, and then maximizing the glide phase for decreased oxygen utilization. For example, in a a cb dive down there is generally a hard kick at the surface to and a hard kick to turn at the bottom to change direction. In addition I try to limit the number of kicks coming up to to conserve air. I can do this because I have a large surface area on my fin that moves a resulting significant mass of water. To return to the bicycling analogy this is like pedalling on a straight patch of road. You want long, slow strokes to maximize the rest period between pedals. With a split fin I would have no rest time and would be constantly making short, fast strokes to produce the same net result. With scuba this probably results in lower total air consumption, especially since they can maintain their aerobic energy pathways longer because the load on their legs is less even though they are stroking more. In freediving, it's the opposite. The slower, harder strokes are more efficient especially since we drop to an anerobic energy pathway for most of the dive.
Please bear in mind this is just my conjecture based on my experiences with splits. Everyone is entitled to their own opinions. Even though I don't think that a freediving split fin would be beneficial I have to admit that I am interested in seeing a prototype for in-water test results.
