The side by side if I recall correctly was one of the earliest designs and could be traced back to German examples which I am sure were copied across Europe. The ones I recall had the utility pouch at the top near the scabbard throat. As far as dirks there is the shell bolster dirk in Foreman's book the had the knives and fork recessed into the scabbard where the shell covered them but that was Scandanavian made.
I have found that what is practical now was practical back then when it comes to making period pieces. With that said I think just overlapping a second piece of leather for the pouch would look correct and it is what I would do.