More theory
I believe I was somewhat misunderstood. I did not mean to suggest using some sort of new bi-fin that is wider. What I am saying is that I always use a dolphin kick with regular bi-fins, using standard scuba fins or long blade fins . . . e.g., Omer Millenium Comps. If you have a good dolphin kick (32 years of competition swimming probably helps), it is second nature to keep your feet together and swim with a powerful dolphin kick. It is so much more powerful than the two-leg flutter kick that I am surprised everyone does not dolphin kick. Dolphin kick is much faster that flutter without fins - just watch any college swim team do kicking drills and you will be convinced, particularly if kickboards are not used and the swimmers are kicking underwater.
Now, I understand your point about turbulence between the legs and between the fin blades. Obviously, it is much more efficient if the kicking surface - in this case the tops and backs of the legs, feet and fins - is smooth and continuous. So I guess what you are saying is that the monofin is much more efficient because it eliminates turbulence and increases the kicking surface. I understand and agree with this logic.
There is another factor to consider, however: the longer the vessel, the faster. Remember that resistance increases in water much faster than in air as speed increases. Therefore, you gain more by streamlining your position - i.e., reducing drag - than by simply gaining power. Competitive swim coaches have learned this in the last few decades, and train swimmers to maximize the amount of time with arms extended in front them spent during the stroke. So, you are faster with your arms stretched above you squeezing your head between them in a tight streamline than you are with arms at your side because you make a longer vessel.
I notice that monofins are short, and I wonder why. Part of the advantage of long blade fins is probably the simple fact that they increase the length of the vessel, not just because they have more kicking surface than shorter fins. So, it stands to reason that if the swimmer can control the fin, a longer monofin would also be faster than a shorter monofin. So finally, this makes me wonder if dolphin kicking with two long blade fins is not close to being as fast as kicking with a short monofin, despite turbulence created by the non-continuous kicking surface.
Certainly, standard long blades give you the option of flutter kicking, or even frog kicking. So, I would certainly advise all divers to learn a good dolphin kick whether or not they switch to a monofin.