How do you shift with two hands on the wheel?
A pro uses two hands because it's gives the best control, but sometimes you have to use one because the other hand is busy or out of position.
This is a never ending argument. I've been cut, more like kissed, with a chainsaw twice. Both hands on the saw both times. Both times were near the end of the day in wicked heat and i was worn out and pissed off. Safety is behavioral, if you get slack, you're more likely to hurt yourself or someone else.
Humans can train themselves to do almost anything safely, including one handing a saw. It's not the technique that's the problem, it's attitude and behavior. Exhausted and hurrying with two hands is more dangerous than fresh and methodical with one hand.