I personally don't think Sephiroth was that powerful (to control Jenova). Geez, you never even see him again (after the Promised Land scene) until you reach the bottom of that crater.

But you know, that would be interesting. Sephiroth's supposed "control" over Jenova makes it seem like more of a tool to him than his supposed "mother." Makes him seem like that much more of a psycho.