I know how the last man rappel. While practicing at a local place, I saw one of the groups using a kamikaze knot for rappel and rope retrieval after the last man rappelled. Kamikaze and Sheepshank are quite the same.
And, How it is supposed to be used: Assume that one of the ropes will be a meter long or less than that which I can fix at the anchor up there and afford to let the piece of rope remain there. And the other strand is what I would rappel on. They did the exact same thing.
The knot looked a bit tricky and I don't quite comprehend the amount of safety it would provide when used with the ropes of different diameters.
Has anybody tried it that way?
Till that day, I had never used kamikaze knot for last man rappel. And, then I tried it with a piece of rope (trusted strength and life) obtained through the same set of rope coil. So, diameter was equal.