Again, I think this is more a product of current ICE AWD systems in which all four wheels are connected in some way. As the front and rear are not connected in any way on Model X (and presumably Model S AWD if it even happens) and can be driven completely independent of each other, I think this will be a non issue. I think electric AWD systems completely throw out a lot of the old rules about AWD in terms of driveline efficiency losses, tire wear, and many others...
FYI, some hybrids (like Toyota Highlander) have electric RWD separate from the FWD driveline.