Thoughts on the little 8L Patagonia Atom or Ultralight Black Hole Slings as that second little carry-on piece to keep things at hand on plane/train if your main bag is in the overhead?

I know this is one-bag but it seems most people take something little as a secondary bag. What is everyone else using?

I have been using this bag for a few years now and I really love it. It folds into itself so you can pack it away very small - it has a very surprising capacity, I can carry pretty much everything I'd need for the day in it (wallet, phone, keys, water bottle, jacket & hat, glasses, reading book, portable charger, snack - I have filled it with this amount of stuff many times), or for example you can fit two wine bottles into the large pocket ;) - I find it very comfortable - it has a load of uselful and well thought out features such as the phone pocket on the front of the shoulder strap the ket clip in the valuables pocket and the small strap on the shoulder strap for carrying/hanging the bag. So far the bag has kept it's quality very well and is undamaged from loads of use, additionally it is covered by Patagonias guarantee meaning they will try to help fix any repairs that need doing to the product during its life.