The first idea of mounting the extruder in the middle makes sense, it just usually isn't worth the extra hassle. The logic behind the idea of a second x-carriage for the extruder is somewhat flawed. Yes, the mass isn't directly on the print head so it can accelerate quickly, but the extruder behind it will still need to go slow and the print head is chained to it via bowden tube so it will also have to go slow as well. It basically ends up being a less effective and more complicated version of having 2 x axis motors (or one bigger x axis motor).

You're also still flinging a heavy mass along the X axis which will shake the Z axis towers.