I believe you can accomplish this with using aim-offsets. Simply create the idle turn animations for 90 degrees, and create aim offsets for the upper body. Then you would want to get the difference between the control rotation (look direction or camera direction) and the actor rotation (this can also be used to control the aim offsets). If the difference is greater than 90 or -90, you can then trigger either of the turning animations. If the turning animations use root motion and are montages, it should work.