I've had myself, my wife & one of our kids in my ENOs (over 425# together) with no problems. If you get the optimum hang angle of 25* to 30*, see how little stress is put on the lines? Under 300 lbs of stress per line on a 250 lb person. You have a huge margin there.
On the other hand, my kids were swinging hard and the nylon Slap Straps were rubbing on some wood about 2' ahead of where the sling was attached. It superheated and snapped in short order.