"a 16 man treadl crane with 2 weals"
The use of two stoppers per wheel would either be simply for more strength, to prevent just this from happening, or alternately having each of the two per wheel engage in an alternating fashion so that the wheel won't have as far to turn and won't build up as much momentum if it does start to turn backwards, also to keep it from failing. With these large loads and a crane built, I assume, out of wood, the working parts would have to be large for strength. This means using larger teeth on the ratchet wheel, and therefore possibly requiring the latter of these two configurations. Either way, when Riddley tripped and knocked over the others working his wheel the stoppers had to take the strain, which they failed to do, causing all the force to be held by the other wheel, and then it too failed. It is a small point, but might be significant because it permits Riddley to be absolved of any blame in the accident, at least by Fister and Straiter Empy. Chalker Marchman's reply puts the blame on the men ... though of course the management would want to avoid blame."