Personally I would use a 50m rope. From memory you don't want to be closer than 30 feet apart. There are two factors to consider:
If you don't have enough rope between people the chances of being dragged in are much greater.
You will need some spare rope to perform the rescue. With only 30m you probably won't have enough.
Rescues where the person who has fallen into the crevasse and is uninjured are usually uncomplicated. However, if the person in the crevasse is badly injured, or unconscious it's much harder. If thee are two of you not in the crevasse, you can probably winch the injured person out.However, this will probably involve one of you going into the crevasse to attach a loop of rope to the injured person to a krab though their harness.With one end of the loop anchored to something like an ice screw, that gives you a mechanical advantage of 2 when heaving.
Edited: 14/03/2012 at 17:35