Remember that the 86 degree specification was taken in a laboratory at the -10db point. What Humminbird Si users have found in actual use is that the beams are wider than this and do overlap. Now water conditions depth and some other factors will influence this but there have been many images posted by Si owners that show this.

“If it is true that there is actually no "gap" then it would seem that all the information needed to display the Di image is already embedded in the Si image...and then it's just a matter of "rearranging" the data into creating the Di image...”
Yep, that’s the way I understand how the Di sonar works.