Unfortunately the stock SOHC4 brake setup is of a very unusual and quirky design (being the first production one) and is not particularly set-up to do a "twin disc". the problem is that the shims need to be put correctly AND the angle of the mounts needs to be adjusted so that the two calipers are not only parallel to each other, but so that the discs are also centred in the calipers correctly. The last time I did this setup, I used two straight edges to make sure that both calipers were parallel, then matched the distance from the centre line of the rotor to the inner face of the caliper on the "second" rotor to the "first one".
This makes both pistons sit at basically the same point on the calipers and everything is just right adjustment wise. If you set it up like this you'll thank me in the long run.
If you have problems with one caliper dragging, you can also swap the calipers and see if that changes it to spot if one of the pistons is sticky on retraction. They'll stick easily if there's a little scum in the seal grooves.
