Depending on the interpretation of the original post, I believe it was correctly answered by the second poster. There wasn't any reference to using two different sensors, or providing for one sensor to serve both purposes since this would be physically impractical requiring moving the sensor back and forth a substantial distance. The only question was why not change the crop mode back and forth mechanically.
Whether or not it would be practical or not in the first place, since any kind of mechanical switching would entail a great deal more complexity and cost, the current software switching "crop" method makes perfect sense. Specifically, as long as the "Auto DX crop" mode isn't selected, this function can be quickly accessed by programming and using the Function or Preview or AE-L/AL-L Buttons plus the Main Command Dial. And as long as "Auto DX Crop" is NOT enabled, the default is to use the entire sensor (FX mode), no matter what lens is mounted. The lens perspective (FX or DX lens) determines how much of the sensor receives a usable image. If you want to switch to a DX crop with either a DX or FX lens, simply use the method above. What could be simpler! (This detail is listed on pages 63 and 64 of the D3 manual.)
OldPhotos "If everyone possesses some measure of this intangible quality called creativity, photography is unprecedented as an outlet for its expression." - Ansel Adams