i think it has alot to do with balance, someone who has been in a skit ski for a long time needs only one ski. but someone who is starting and might not have the balance figured out so i imagine it helps with the stability of the whole rig
Opinions are like nipples, everybody has one. Some have firm points, others are barely discernible through layers, and some are displayed at every opportunity regardless of whether the audience has stated "I am interested in your nipples" or not - David Thorne
Its called a bi-ski. They are much easier than a mono-ski. Riley's brake might be too high to use a mono-ski. It requires lots of core strength to use a mono-ski. Riley's brake might be above that or maybe he is just learning. With a bi-ski all you have to do is lean and you carve a perfect turn. With fixed outriggers people who can only move their heads can go out on a bi-ski. They just need someone to ski behind them with tethers to help control speed.