DrDoc - 5:34 pm on Aug 19, 2013 (gmt 0)
I really don't see the need for a separate rule based on screen size. You could simply make the width fixed (or a percentage, also setting
min-width to 512px) and float the panels left.
If the width (or effective width) is 1024px or less, they will behave like you describe. Otherwise, they will line up side by side.
Without a brief sample of the difference between the two, it's difficult to say ...