Just to make things clear: you CAN, and by default actually DO map the tablet to both displays, there is no problem in that.
But: mapping the whole tablet to both displays not only effectively reduces the per-screen horizontal resolution (and hence the precision) but also distorts the aspect ratio of the screen/tablet relationship (shapes appear squashed horizontally).
Especially when doing real paintwork, this is rather annoying.
It would be nice to have a means to switch the mapped area to different monitors: the whole tablet is mapped to one single display, and with the use of a button (for example, you could use the Intuos' tabletbuttons for this) you switch the mapping to the 2nd monitor.
Subdivide and conquer!
Free your mind, and your ass will follow!