What I can confirm is I've traveled with my young child, and I use my KID's credentials on one of my devices and MINE on another. (So I can use two at once for free.)

So yes, I think what you say is accurate. I've thought about it as a security issue. Not sure a good solution exists except to say, CX allows device switching (good!) and you are auto logged off one device when you log in with the new one. So the other person would have to consistently log back in as you. Not perfect but it eliminates the biggest potential problem - a "one and done" scenario where you can only use the freebie on ONE device.