Even if "clone" is a botch by sony, there are two distinct points that are unexplained of this is true.

1. the existence of 2 sephiroths
2. the connection between cloud and sephiroth (primarily illustrated by mind and muscle control)

if you choose to specify the second as bull, I would ask why sephiroth wouldn't just control someone else, say for example, tifa, to get what he wants, or better, make cloud do what he wants by blackmail. There is some distinct connection or reason for his exclusive control of cloud.

This thread has a lot of contradictions. Since many of us haven't played in a long while, and probably have fleeting memory of the complex story, I suggest someone finds a website and provide some evidence via a link or reference.