When fitting individual bar risers, they can be difficult to tighten without them rotating slightly because you can't hold the mounting bolt when the bar is in place. If the risers aren't pointing exactly forward and perfectly parallel, the bars won't sit properly in them. I have tried just tightening the nut, but there wasn't enough friction to prevent the bolt turning before the correct torque setting was reached.
A short piece of 29mm diameter aluminium tube with a hole drilled through has made the job simple.
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Just snug up both risers, but not so tight that they can't be rotated, put the tube in the risers making sure the risers are straight and the hole in the tube is over one of the mounting bolts. Tighten down one cap to keep everything straight, then slip an allen key through the hole to hold the bolt while you torque it down. Now remove the clamp, reposition the tube, and repeat the process on the second riser.