I never gave it much thought before but looking at the various Tarptents from Henry Shires I got the idea that you could easily add a floorless inner tent to create a double skin tent, in fact Henry provides something similar, a Clip-In Liner for the Double Rainbow.
That got me thinking about double skin tents in general, why have the groundsheet and for that matter the mesh doors attached to the inner tent? Imagine if the groundsheet and mesh was permanently attached to the fly to which you added the floorless inner. You could use it as a single skin with full groundsheet and midge protection if you wanted to reduce weight and simply add the inner when needed.
There doesn't seem much to be gained by attaching a groundsheet, which will probably end up wet and muddy, to the inner tent which ideally you want to remain clean and dry but thats how every tent I've ever seen has been designed, any ideas why this is? wouldn't it make more sense to have the fly/groundsheet together as both are likely to be wet anyway.
The best reason I can come up with is that maybe historically tents were canvas/cotton with a sewn in ground sheet to which they later decided to add a nylon waterproof 'Fly' and thats how they've remained, just conventional. Is there any logical reason why it couldn't be done the other way?