IMHO I never could figure out the why for a double bag. I have never had a need for a double bag. I carry only the minimum in my pouch to load and maintain my fowler or rifle. Anything other than that, I carry a small haversack on my opposite side.
Allows you to keep your flints, turnscrew, worm, and tow in the back, and keep the balls loose in the front? I have a single compartment bag and keep everything in its own separate sub-container, a la Mark Baker, but have found that I spend a lot of time trying to get a patch out of the patch bag, then hold on to it while I fumble around taking the stopper out of the ball pouch, getting a ball to come out of the pouch, then replace the stopper, and replace the ball bag. Lot easier and smoother (and faster) to just reach int and grab a loose ball off the bottom of the bag, but you can't do that if you have other stuff in the bag.
Next bag I make will definitely be sub-divided, so I can stow the flint, tow, and tools in the rear section and leave the balls loose in the front one. Might even put a pocket in particularly to carry patches, if I use a wooden box on the rifle instead of a brass one.