I'm not quite convinced Yozora is a fusion. As in, the two of them fused together or something. It's a common theory, but... I'm not on that train yet. One obvious question would be... Why? Another question would then be, how did Yozora get his red eye? Both Sora and Riku's eyes are blue/bluish-green, so naturally you'd expect a fused version of the two to have a similar eye color.
Another thing is, why is Yozora (a video game character in Toy Box) a real person that's showing up in The Final World and in Shinjuku? If Sora and Riku fused and became Yozora, how does that play into it? As a matter of fact, does The Final World apply to ALL Worldlines? Or is it just the one we know?
Yozora means Night Sky, and Sora means Sky, so there's an obvious connection. Similarly, Noctis's full name is Night Sky (or a variation of it), plus the guy looks extremely like Noctis by design. But he also looks like Riku.
I won't say it's impossible that they fused, because there's definitely instances in which characters have fused before. But right now, I'm feeling like he may just be an Alternate Sora counterpart in a different worldline due to the similar names (and the KH1 opening line thing). Magia and Aegis seem to be Yozora's counterparts to Donald and Goofy, so another theory I've seen floating around is that Yozora is from a worldline that's more "realistic" than the one we know about, and Yozora is the protagonist in that universe (which explains why Sora gets confused for Yozora).
But this one I'm not too sure about either, because in the Verum Rex commercial, we don't see Yozora with a Keyblade (but there's a Keyblade patch on his shoulder sleeves).
In the end, there's clearly something there that connects Sora and Riku to Yozora, but of course, there's not really much information out there to say exactly how.