Even with fusion, you would have to choose who the dominant personality is. The answer being Luke.
What about the option: Fusion - but with entirely new personality?
It means you never spent any attention on the plot or the sidequest concerning the fusion.
You mean that Sidequest that is rather complicated to understand at times, but eventually, if you think about it, totally means that the conditions for That Sidequest Fusion and Luke-Asch Fusion are completely different? That one? Yeah, aside from the conditions being different, it does not nullifya fusion option, so I say there is still a possibility that it is so.
Cloud and Sephiroth - battling each other since '97 for your amusement Give it a rest already!