Sorry to go back to the previous discussion about how to preserve the Vader being Anakin surprise but I just wanted to add my idea here. If the duel between Anakin and Obi Wan could be editted so that the two characters both appeared dark, so there is very little distinguishing them in the distant shots. And then one of their lightsaber blades could be changed to red, so that is our way to tell them apart. Much like in this artwork:

Then I think by removing all of the close up shots of the actors faces, and adding a few shots of a solitary Obi Wan, it could possibly be made to look like Obi Wan is viewing the fight from a distance.

If the idea of Obi Wan's second apprentice were used I think it could be made to look like Anakin and that apprentice are fighting. The apprentice would be shown to use a red bladed lightsaber, and Obi Wan, unable to reach the two in time would just see one figure holding a red bladed saber standing above the other person, who would be holding on to a ledge or something, before being pushed off. The murderer would then walk away. Obi Wan would later reach the place the person fell and retrieve Anakin's lightsaber from the ground.

As long as the second character was generally the same size and had similar hair and outfit to Anakin, just so they weren't identifiable from a distance, I think it could be enough to sell the lie.

Also we know Ady wants to include newly filmed footage, and I think the existing duel has enough elements that it would only take a few new shots to tell a version like this. For example a shot of the new character, and then one of them walking away from the ledge and picking up a cloak or something as they walk away, add in a few new lines from both characters and it would be a new scene with relatively little effort.