I've noticed that every now and again, as I'm preparing to fight bizarro sephiroth, all the other party members will be blown away and I'll be left with only one party to fight sephi. I've been able to do this on several occasions, and I've gathered that this happens if you "struggle" in the fight against Jenova synthesis, but I'm not sure exactly what the conditions are... Is it a time thing? I can't imagine that it is, as every time I achieved this, jenova didn't even live long enough to cast ultima, but then again, I used KotR a few times, and that's a pretty time consuming attack.
Anybody have any ideas?