Balancing out the braking forces is definitely a good thing, but most DH forks should be sturdy enough to make the twisting very minimal. Also, such a design would allow for greater heat dissipation, meaning that you wouldn't have as many problems with fade and overheating the fluid. However, the same effect could be achieved by simply increasing the size of the rotor on a conventional system....oh wait...they've done that! Eight inch rotors (and maybe larger, as bikes get faster) give all of these benefits without the downside.