Technically, ROAR says it would be illegal, but only because you're using 2 servos. If you somehow had front and rear steering with just one servo, it'd be legal.
Which is silly considering they used to allow monster trucks to run dual steering servos, back when we used to race those
I think it could be an awesome setup, I may have to try one out myself. With mixing, I should be able to have the rear steer only when I hit max travel on the wheel, like for a tight 180. All other speeds would be normal steering. Even better would be a little crab steering at speed, definitely worth trying.....
There's a thread on here somewhere where a guy made his TC 4WS, I think he liked it too. I'll try to find it.
From the rule book-
4.1.9 Except in 1/5 scale class vehicles, radio systems are limited to the use of two channels, operating two servos, or a combination of one servo and one electronic speed controller. More than one function is allowed per device, but a servo is limited to one output shaft. Control is limited to forward, brake, and steering. The use of reverse in competition is not allowed. On 1/5 scale vehicles, three channels and more than two servos may be used, but control is limited to forward, brake and steering.