The biggest and best reason…
Paul @PDWhite listed ‘truely zero torque’ as the first reason for using multiple motors, but didn’t really elaborate on it. A lot of pilots, especially those with previous experience of paramotors, will get that straight away but it occurs to me that many will of not noticed the ramifications. So here’s a VERY simplistic explanation of why zero torque is such a big deal, aimed not so much at most of the people here but at those passing through or only just starting to look into becoming flyers.
There are two ways to turn a paraglider. Pulling on a brake line, those are the lines attached to the trailing edge of the wing and held in the pilots hands, or weight shifting. In weight shift the pilot leans over increasing the wing loading on that side. (Though a paraglider looks like one skinny parachute it’s actually two inflatable wings connected by a bellows section. All the lines on say the right wing collect together through some straps called risers and are fixed to the right side of the harness slightly higher than the pilots hip. The left ones all go to the left side. The brakes are lines connected to a wings trailing edge, the back, again left hand to left wing only). When the wing loading is increased that wing slows thus if the pilot leans hard to the right the right wing slows a little and the left wing speeds up by a slightly greater amount resulting in the paraglider turning direction to the right. Note that no brake has been imputed which means that the wing has not been distorted resulting in a more efficient turn.
Now add a motor. In the diagram below the black arrow represents the turning propeller. Its beating the air with around 12hp. Remember back in school when they said every action results in an equal and opposite reaction? The prop is not just pushing air behind it resulting in a reaction forwards (which we want) but is also spinning against resistance which causes the motor and its mounting to the harness trying to twist the opposite way, red arrow (which we don’t want). This force translates into a pull through the risers on that side, green arrow. Effectively it’s a weight shift input resulting in a turn, yellow arrow.
Historically, (and I’m an old fart who started paragliding in 2000) there have been several systems to try and negate the effect including: Variable hang points (the point at which the risers are fixed to the harness). Reflexed wings that respond poorly to weigh shift. Motors mounted to produce offset thrust. Trimmers on the risers to cause one wing to be a bit faster than the other or to effectively raise or lower the relative hang point. All of these methods are ways of causing the craft to turn by about the same amount, but to the other side, that the motor is causing a turn thus negating each other. Aside from the loss in efficiency this produces another problem. Generally a paraglider flies at one speed, it’s trimspeed (yes I know about speed bar etc but this is a Janet & John article 
when the throttle is opened a paramotor does not fly faster, it climbs. When the throttle is closed it does not fly slower, it descends. So if the torque compensation is set to fly straight and level at a certain power setting then it follows that when you want to take off it’ll try to turn one way and when you come into land it’ll turn the other!
On Paul’s design two of the propellers are turning clockwise and two anticlockwise so any resultant torque effects are balanced within the motor framework and transmitted no further. This also means that the harness does not need to be so rigid and can be tuned to allow the pilots (and only the pilots) weightshift input to cause more control (lower and wider set hangpoints). It used to be very difficult to fly a paramotor as a paraglider, gaining altitude on the motor then thermalling XC, but with a torqueless setup it should be possible.
Wow. This got a lot wordier than I’d intended! Apologies to anyone who feels I’ve been stating the bleeding obvious and missing out a few important bits, but as I said previously there will be people hitting this forum who share the dreams but don’t fly… Yet.
and have no efficiency at low airspeeds. They’re designed for high airspeed applications.
