So with dual action you'd have to undo the bolt tighten the cable redo the bolt. Whereas dual action would require me to also adjust the non-actuated pad with the bolt on the side? I've only briefly used mechanical disk brakes but never got on with them, this might be what I've been doing wrong.
No, with single action you adjust the fixed pad and move the caliper. With dual action you adjust both pads. You should avoid using the cable adjuster due to the change as the arm moves of the ratio between cable pull and pad movement.
So with dual action you'd have to undo the bolt tighten the cable redo the bolt. Whereas dual action would require me to also adjust the non-actuated pad with the bolt on the side? I've only briefly used mechanical disk brakes but never got on with them, this might be what I've been doing wrong.