atty-guy :"My struggle here is that I can't see when there would be a convergence of a shortness of time and a need to go very light. The only scenario I can envision would be if I were being chased by someone. Am I missing something here? "

Ah yes,I should have mentioned-
I live on Vancouver Island and the biggest threat the we face out here is earthquake,so the main scenario that I plan for is a major earthquake. I live in an older apartment building surrounded by other old apartment buildings, in a suburbanized area and chances are if the "big one" hits - i'll have to very quickly evacuate the building,the immediate area, and possibly very quickly leave the city. Plus I have 2 cats so once i manage to exit the building i may have to sacrifice bulk for cat!
I've tried to plan for worst case- apartment buildings collapsing/on fire around me, I may have to make run for it and the larger pack would slow me down. I may be injured and not able to carry a full pack and yet still have to leave as quickly as possible. I leave the main pack slightly open so that i can easily and quickly remove the smaller bag , then if i have to i can make the decision in a split second-grab and run- and if it's at night and I'm asleep I am probably not wearing something with a lot (or any)pockets to put stuff in to( the majority of west coast earthquakes hit at night or very early in the morning. I also keep a flashlight,a pair of sneakers and crowbar under my bed- imagine no lights ,a lot of broken glass from windows and a newly off kilter door frame complete with wedged door- none of this stuff will do me any good if i can't even get out of the bedroom!) , I also keep a pair of light canvas sneakers,pants and shirt in a large ziploc just underneath the secondary bag just for this kind of night time situation-though if i can only grab the secondary bag, I can cover up with a couple of (non-see thru- I checked,ha) heavy duty garbage bags that are in it.

Time wise, if my apartment is being shaken to pieces ,taking a little more time to grab containers and stuff pockets could mean trouble- a falling beam to the noggin will could kill me just as dead as a bullet or a bomb. With both packs i can take my time or not, as the situation demands.

I 've planned for emergency situations that are most likely to crop up in my part of the world which tend to more of the natural disaster type of thing, with you being out in D.C. I imagine that you're plans are more geared toward terrorist attacks and you may have that extra few minutes to pack up (just a guess),so what is (i think,ha) right for me -second bag etc..- may not be right for your most likely scenario.

Oh,also (man am I long winded today) my secondary "bag in a bag" is very very light (10$ nylon walmart special,about the size of 4 vhs tapes) so as to add very little weight to my main pack.

Hope I answered your questions .