I used to have a tamrack aero 70 for my canon XSi with 28-135/3.5-5.6 lens, waterproof P&S camera, laptop, large tablet, and lots of other things. That thing was always loaded over 20lbs and never had a problem along with my regular carry on going through airports. I've expanded since and been looking for something to fit 5d3, 24-70/2.8, 70-200/2.8, 35/1.4, flash, 15" laptop, and other small things for a day outing as well as small enough to accompany a regular carry on for flights.
I really like the crumpler karachi outpost bags (not the newest ones, but previous ones). The small doesn't seem to fit 15" laptop (can anyone confirm please?), the medium one might fit but not sure if it looks too big as a second item. The thinktank airport hard drive seems to work too, but looks boxy and also not sure if it's too big. What other options??
If you use a camera bag with laptop as a second item along with your regular carry on, what are you using? What do you think would work?