The following describes how I understand it (i.e how it was explained to me):
You have 2 767's flying in formation side by side, one very heavy and the other very light. The engines on both aircraft are shut-down at the same time, and the aircraft are both glided to a dead stick landing at their respective best glide speeds.
Both aircraft will glide exactly the same distance, and should land right next to each other. The heavier one will land first however due to it's higher groundspeed (due to higher glide speed) and higher descent rate, the lighter one will take a bit longer but won't get any further.
Regards,
Gordon.