In all the lists of Bug-Out-Bag contents, I never see mention of having a duffle/backpack/BOB inside your primary BOB.

Most BOB's are packed full, with no room for additional gear.

A secondary bag would be useful for things you collect along the way, without having to discard items you are already carrying.

The main reason for a secondary bag, though, would be sharing the load with someone else you might be with. If you and a buddy find yourself having to hump out bf being off the road in the winter, you'll probably be depending on just the gear in your vehicle. Having a second bag, means you can split things up.

A second bag would also help if you primary bag were damaged badly.

Of course, you can fashion something out of a tarp, as a secondary, but I'd rather have a second bag. If it turns out you don't need it, and you want to save weight, it can always be discarded/burnt/left behind.

Good idea, I have a couple of very thin backpack type bags, not sturdy enough for a lot of wt but light and packable enough to bring along.

I have a milsurp willie pete bag packed in bob that is for waterproofing in case of water crossing that would work.

Acutally, my BOB rests inside a duffle bag (actually, and airborne kit/parachute bag).
All my Army duffles are packed with longer-term gear for grab and go.

Kifaru pod for you MOLLE types.

My Kelty pack has a detachable pack that goes on top when it's all put together.  I also pack the stuff that's inside in two military wet weather bags.  My GHB is always in my car and I've also got another small hydration pack with some basic stuff in it that I carry with me when I'm walking the dog or going on day hikes.  Those are the handouts.  

Quoted: Kifaru pod for you MOLLE types.

res---sist------ urge to----------

your bob should never be filled to teh brim,if it is you need a bigger pack or pack less crap

I have some gallon ziplocs, and usually some plastic grocery bags in my pack....sometimes I stash a rolled up dry bag in my pack.  Contemplating getting one large enough to stash my pack in considering how awful wet the weather has been.  

Quoted: Quoted: Kifaru pod for you MOLLE types. res---sist------ urge to---------- your bob should never be filled to teh brim,if it is you need a bigger pack or pack less crap

Don't worry, after about 5 miles with an overstuffed bag there'll be a trail of discarded shit left behind

when I go hiking I take a light weight Jan sport bag, I use it for day hikes out of base camp. When I was hiking in the marble mountains( my first real hiking adventure) I found out that a military but pack worked great for day pack, I just took the straps off of my alice pack it worked real well.

I also received an ultralight unstructured mini-pack as a gift.  I might roll it up and stuff it in the main bag as a backup.

My Bob rest in a duffel with a change of clothes and boots.
I have a mesh foraging bag with cord straps in my BOB.
Gotta stay focused on less is more...at some point I'd look like that pic. of the overloaded Pakastani Unimog.