Epic? Really? Why thank you, glad you enjoyed.
If we're saying they both made into SOLDIER, then there are some unknowns we have to look into. I'd say they're both considered 1st Class, since its been noted they have very similar potential and such, and Shinra needed someone to fill the top slots after Genesis and Angeal took off. Zack probably wound up with the Buster Sword since he was Angeal's Puppy, whereas I see Cloud (who is a bit younger) getting Angeal's lessons second hand from training with Zack. Thus Cloud has either a standard SOLDIER sword, or perhaps picked up a Force Edge or Mythril Saber somewhere so that he can have his own weapon after becoming 1st.
Now, in those circumstances, I find it quite likely that Sephiroth, as he was during the Nibelheim incident, would get killed outright rather than tossed into the Lifestream. With a full compliment of enhancements for both of them, Cloud and Zack would arrive at roughly the same time. When Cloud went to move Tifa aside, Zack wouldn't just run on, because he'd be aware he has powerful backup in his longtime friend. He also wouldn't want to deny Cloud his chance to lash out at the man who killed his hometown and such. He'd probably be compelled to remind Cloud of his SOLDIER honor, then they'd go after Sephy together.
From watching him spar with Genesis and Angeal, we can reasonably say Sephiroth would hold his own for much of the battle, but that was (mostly) a friendly sparring match, with only Genesis having a personal beef with anyone. Cloud has, as we've been over, a massive rage thing going at Sephiroth. Zack is a little less involved with that regard, but he's Cloud's best friend, he's going to share some of that pain and rage with him and want to help him get justice for his mom and sweetheart and such.
I picture them taking a moment to discuss strategy, remind each other how strong their opponent is, maybe have that SOLDIER honor reminder. Sephiroth exists Jenova's chamber with her severed head in hand. Because Tifa is in the pod room, its not even close to a safe place for a battle, so the first thing Cloud and Zack do is to move the fight outside into the Nibel Mountains. They both take a number of wounds, but because they're supporting each other Sephiroth never has a chance to completely crush or impale anyone before the other is harrying his flanks, forcing him to turn and defend while the other recovers. Just because it would be cool, I picture the finish as some kind of huge double Braver that cleaves him into four pieces.
And that changes the entire picture, of course. Because they're not almost dead, neither Cloud nor Zack winds up involved in the Jenova Reunion experiments. Disgusted by what Hojo is doing, but wary of facing the Turks and Shinra military in their battered state, they'd escape from Nibelheim with Tifa and Zangan. The battle that claimed Zack's life in the OG flashback/CC ending never occurs because they've all left early with no need to escape from their stasis capsules years down the line. They may have become mercenaries, as per Zack's plan from the OG, or they might have just gone directly into joining the second AVALANCHE when Barret forms the organization. The additional muscle from Zack could have helped prevent the deaths of Biggs, Wedge, and Jessie, the fall of the Sector 7 plate (since Cloud and Zack are both alive and probably both remember Reno, this could be rather emotional) and so on. Plus Sephy never comes back to life, and thus the entire battle is against Shinra without Sephiroth and WEAPON getting involved. It becomes an entirely different story. And kind of a cool one at that, now that I think about it...