You might want to look at Kifaru packs. They support something they call the
piggyback system (docking system). From reports in their
forums people who try this with a second full size pack are not thrilled, but they have a small pack/pouch called the
E&E that would probably be much more acceptable.
The other good thing about the Kifaru packs is that unlike most military packs, they have a frame. Personally I think this is an importaint feature.
I'd be interested in your experiences if you try this. I've been considering getting a
Marauder and pairing it with an E&E.
That said, I currently use a two part config like you describe. It's more of an EDC rather than BOB. I use a Maxpedition Fatboy Jumbo and a Kelty Redwing 2400 pack (medium sized daypack).
Having the Jumbo with my most importaint stuff works very well. It is small enough that I'm willing to keep it with me pretty much all the time. The BIG downside is the inability to dock the Jumbo for carry from one place to another. The Jumbo is also an awkward size and doesn't easily fit into a backpack and then it is difficult to access things in there.
I find my existing system works well for me, but having the ability to dock the two would be much more optimal.
-john