Osprey has a few combinable systems with the Daylight Daypack and will be releasing the "Ozone Duplex" 2-in-1 bag system this fall. It seems like the smaller daypack is intended for heavier gadgets on the back while the extension unclips for side messenger/brief carry.

Do you need both game units, both Kindles, and Bose headphones easily accessible until you're at the airport/train/bus station? Have you considered putting some of the gadgets into a small packing cube and stowing in the main backpack, close to your back, to offset some of the weight while walking? You could take it out before boarding/sitting and put it back for longer treks.