Briggs and Riley has the BRX series where the backpack clips onto the carryon for ease when you are places but the separate so one is your personal bag and one is the carryon. I really like the design of this. However, I need to measure it again as I am concerned that the wheels portion is 12 inches deep on the carryon and could be a problem putting it into the overhead -- but I will look at it again. The carruon does expand but it is not a zip around expansion -- instead the top side zips open to be the same width as the wheels section.

Eagle Creek makes a similar setup but I do not like the configuration of their suitcases as well. However, people who do love them so it may work for you.

I usually travel with a backpack and a carryon but the backpack I have will not sit on top or clip onto the carryon -- and it does get heavy so I am looking for a different configuration.