Proof by reductio:

Assume I am both R and S.

That is, I = R, and I = S (where "=" means "is literally the same thing as")

So, R = S (by symmetry and transitivity of "=")

But, R S.

Contradiction!

Hence, I am not both R and S. (by reductio)

This is proof of premise 2 of the overall argument.

Defense of "R S": "They're going to live completely different lives. They're going to be as different as any two people are." (P 298a)