Based on Tmar's photo as well as the exploded assembly diagram, I would disengage the spring ends from the caliper arms first. Then hold the unthreaded shaft of "D" with a small locked Vise-grip or the similar Craftsman version. I have a long nose narrow-jaw one I bought for adjusting old Brooks tensioners. The latched Vise-grip needs either a muscular assistant (cats not recommended) or a real or improvised bench vise. Take careful note of the play between the two arms and if they are able to rock on the shafts of "D". Then unscrew both nuts without losing them or the washers. When you separate the two arms from each other, don't lose any of the washers, or the ones on the interface between the second arm and "D". Those will be really important to re-assembling without play or binding.