For downhill and all forms of trail riding (freeride, enduro, all mountain etc) the prefered way is 1 finger (index) on each lever. Anything else can be dangerous because the handlebar needs grip. And having a lot of fingers on the levers and not on the bars reduces grip. If the brakes are not powerful enough for dealing with this then they either need fix or replacement to proper brakes for this type of riding.
Also, riders position the levers closer to the stem (leaving 1 to 2 inches space between the grip and lever clamp) in order to enforce that only 1 finger can go on the lever. This also helps the finger pull the lever from its most powerful spot.