i know a guy who can rep 4 one arm chins from dead hang position. you dont necessarily need to be very light instead you need to be able to chin approx 185% of your b/w (some study proved this) with two arms.
Also its a ridiculously low percentage of ppl who can achieve this. Somewhere in the region one in every 50,000 or something liker that. It also take a lot of time to train for it as you have to build up to it incrementally.
One easy way to do this is to hang a rope over a chin bar holding onto the bar with one hand and the top of the rope as close to the bar as possible. over time you simply and gradually lower your hand position on the rope down (approx 22-23") for the average person and then you should be good to try it.