I have lost the plot here, going now, to lie down in a darkened room.
Don't go to the darkened room just yet. Ed has produced an ingenious solution except that the inner part of a niju habaki is essentially a slimmed down version of any other habaki. In other words it is too rigid to flex by the ammount required as it has edges as well as faces and they are of the same length at the front of the habaki. True the insert on the ha side stops short of the front edge where it meets the ha machi but in a habaki in good condition there should not be a split. So very little flex.
I have spent some time looking at habaki and here are some thoughts. As I already suggested the significant fit is the point where the habaki meets the machi and is effectively pushed against them by the action of the tapered mekugi. If a sword is Shinshinto or newer one would expect the blade to be in near original shape and the habaki to fit all round quite nicely. One might expect the same in an osuriage sword, depending on the amount of polishing that it has undergone since suriage was done.
When a sword has undergone a lot of polishing then the kasane of the blade becomes thinner than the kasane of the nakago. In the cases I have to hand and can find illustrated, sources are not very good at showing this point, the habaki fits beautifully at the machi but it does not fit the faces of the blade at all well. For an excellent illustration have a look at the sword that Nick has recently posted, look at the shot of the blade in koshirae looking toward the tsuka from the blade. http://www.militaria...tachi-for-sale/
The blade I referred to in an earlier part of the thread has a stunning silver niju habaki which passes over the nakago and seats firmly but there are quite visible gaps between the leaves of the habaki and the faces of the blade. Once the tsuka is attached and the mekugi inserted the fit in the saya is amazing and there is no wobble.
I have not had the opportunity of seeing Brian Tschernega's work in hand but I suspect that a view down the blade toward the habaki will confirm that he has very skillfully made the habaki a tiny bit more generous as it fits the nakago and relying on an excellent fit at the machi. We would all agree with Franco's point:
a properly made habaki whether one piece or two piece, should in fact slide without contact over the nakago, and still fit snug and firmly onto the finished sword when seated as well.
This confirms that we don't need to start inventing theories about hidden craft skills and the magic niju habaki, they are amazing pieces of craftsmanship, usually under rated and often overlooked. Perhaps all such craftsmanship should be regarded as a little magical.
All the best.