Funny you should bring this thread up, as I just spent the last few nights going over my own rear multipack option.
I was running an MBSS, attached with the molle on the back panel of my weesatch. Weight distribution was awesome and it was secure. BUT I was very limited on what I could carry, and even when it got a few things in it, it got bulky.
Biggest negative was one I put my 3L camel bladder in the dedicated mesh pocket, that really took over 80% of the entire storage space. I was limited to maybe a clothing layer and extra gloves in the main pack before that was stuffed, and then some snacks, extra ammo or battery, small tools in the smaller pouch.
What I just ended up doing was taking that off, and modifying my small US Woodland pack, one just like this
to take its place.
Now the biggest reason I decided to go over my rear pack options, was for 24-40 hour ops where I end up carrying my large 3day out with me, this was really uncomfortable carrying that ON TOP of the MBSS on the back of my rig. That reason + limited storage options made up my mind to make the swap.
Essentially what I did was add some compression straps to this backpack to help once its loaded, and it compresses down to just about the same profile as the MBSS but because its a bit wider, it carries more in the same amount of rear bulk.
I also added 6 Fastex clips to attach it, and also gives me the option to quick remove it. 2 on top, 2 on the bottom and 2 on the side. Keeps it nice and stable.
SO for day events I can carry 3L water plus some kit in the main pocket, jacket, goretex, ect. Plenty of room in the front pocket for food, ammo, batteries and tools plus extas, compressed down nicely.
For 24 hour ops I can remove it to carry my 3 day pack, and with the clips, I attach it to the side of my 3day to carry everything out much more comfortably to a CP or hide, drop the pack, set up camp, and throw the small one on my rig and off I go.
Should work great for my own personal preference moving forward.