esoomenona, I though about that before I designed the concept GH70 case (joined GH60 + GHkeypad), but the downsides are exactly what regack mentioned. Although having that extra top cover could lock the two bottom piece together is more versatile, here are the "problems":
1) The center divider is really thick because it has to be a combination of both walls
2) There will be 2 separate USB cables going to the computer. If you don't want this, then there needs to be 2 holes cut in each case to have a joined cable, and this is just ugly if the GH60 and GHkeypad are used separately.
3) You'll have to get 3 top covers. Two for the two independent ones in case you want to use the separately, and the large cover to join them
4) The rounded fillet corners in the separate bottom piece force you to also make a rounded cut at the corresponding top cover if you want it to look nice. Even then, that rounded cut in the top cover looks a bit weird when the chamfers are also added. I actually don't think it's even possible to machine that rounded cut.