how to use the add-on strap?

Of course, without seeing it, it is hard to offer directions. BUT the strap I have for one of my rolling bags attaches to the top handle and then around the handle of the second bag so the bag kind of rides by hanging down the front of the first bag. That way the weight of the second bag rides closer to the wheels instead of weight of the bag being higher on the handle. I have seen bags with a large plastic clip for the strap to click into instead of wrapping around the handle of the first bag.

PS -- I just looked at the Steves' current rolling bag. It appears to have a plastic clip under the top handle. So clip the male end of the strap into the female end clip on the bag. Should be obvious.

It works as Frank describes. What happens is the weight of the smaller bag counterbalances, somehow, the weight of the carryon so that it feels almost weightless as you are pulling it along. When a second bag is over the pull handles of the carryon, it is much heavier. However, the second bag needs a grab handle that is tight to the bag, versus an over the shoulder strap, etc. so that the bag will hang without dragging. Some people have bags that are two big as that second bag so it doesn't work. That being said, my husband had a 20" wheeled Jansport backpack for his second bag on this last trip due to his camera equipment (and meds) and it worked fine due to the tight handle being in the correct location for the hang a bag strap. The RS Euro bag and others of that type work well for the second bag.

Thank you both for these suggestions!