***As long as the extra bag packs IN the Onebag, I think we're still according to Onebagging Hoyle here.

I carry a folded up Arc'teryx Index 15 tucked inside the straps compartment of my Osprey pack. https://arcteryx.com/us/en/shop/index-15-backpack
Still only one bag when I get on the plane, but at hotel, I dump the larger bag, pull out the "day bag" for around town use. That way I'm not emptying everything out of my big bag to just haul some shopping, camera, jacket around with me. I did a week in Hong Kong with that setup, came in really handy.

Eddie Bauer has a superlite packable backpack that's quite good for this purpose, fo' cheap.https://www.eddiebauer.com/p/8230016...ize=ONE%20SIZE