I don’t understand the terms “twinrigger” or “fly shooter.” The photo in the link shows the starboard side trawl door hanging over the side. That is a very high aspect trawl door which suggests midwater trawling, or that they “fly” the trawl doors above the bottom while bottom trawling.
The PDF describes a more powerful center trawl winch, so I’m beginning to believe that they do tow two nets at once. Probably why it’s called a twinrigger. Apparently, one set of trawl doors spreads both nets, and the inside wings of the nets are both attached to the center winch. I do not see why anyone would midwater trawl with two smaller nets, instead of one large net. I can see how it might make sense to bottom trawl with twin nets.