This thread is funny.
The OP said that we cannot use Sephiroth fused with Jenova, but everyone is talking about the scene with the Zolom, where Sephiroth WAS fused with Jenova.
He was using Jenova's body to kill the Zolom, exactly like he was using her body/head to fight Cloud in AC.
The only difference between the Sephiroth that killed the Zolom and the one in AC, is that one of them was empowered by the Negative Lifestream, and the other wasn't.
Not to mention that, by the end of FFVII, we can say that the real Sephiroth(at the Crater) was "fused" with Jenova:
Information about FFVII Sephiroth, from the Compilation Ultimania - "He synthesized with Jenova's cells to complete his revival"