The way I had been rolling was to have my official carry-on be my computer backpack, and have another bag that could be carry-on if it needs to be, but I have been just checking it (I could fit the backpack in that other bag if I needed to take a bigger bag as check-in, such as for skis). Anyway, I only carry a few days worth of stuff in it, to get me through the time of putting my stuff into storage at my "home away from home" and my real home - but as my "home away from home" can be icy, I have to carry snow shoes (actually just trail shoes with a really good tread) and cold-weather stuff like a ushanka. I also carry a "purse" (i.e., a bag for my 10" pad)

Anyway, it seems to always be a very tight fit, and I was thinking about getting some type of a sling bag that I could wear in front, even while flying. I don't think I need a whole lot of extra volume - about 400 cubic inches would be fine. The important spec is that I can wear it around my front while I wear my backpack on the back. And of course, it has to be something that the gate staff don't call a "bag".

EDIT: It looks like this will do nicely: