Birth by Sleep Observation on Riku -Spoilers for BBS and KH1-

So end of BBS we have Terra and Xehanort sharing a body. At some point the apprentices get killed in an experiment and we end up with heartless "Ansem" among other things. This heartless then posses Riku. You've now got three keyblade wielding hearts sharing one body. They then proceed to make 6/7 of the keyblade of hearts, the only keyblade powerful enough to open Kingdom Hearts alone (perhaps excluding the kye-blade since that is unknown but pretty powerful). And you had to fight that as a boss? How was Sora ever able to beat that?

I also noticed that Riku only used Souleater, until the time of his possession. Does the passing of power have some requirement to meet before it takes full effect? And if so has it taken effect at all yet? Riku still has a shadow of the heartless of Terranort lingering inside him. Could he just be using one of their keyblades?

 

I didn't think that Riku allowed Xehanort's Heartless into his body until that one cutscene, after Sora reclaims the Keyblade, where Riku is approached by the cloaked figure and gets the new power. He was able to claim the key from Sora at first because Terra had passed the power to him, but he didn't get the Keyblade made from the Princess's hearts until he truly opened his heart to Xehanort's Heartless.

 

Terranort in my eyes is like Lord Voldermort. He finds one way or another to ressurrect himself using his heart, heartless, soul, keyblade, body, etc, to come back to life. The specifics like Nomura usually likes his fans to do is make them up for themselves. It's one of the only reasons I'd still be able to understand the plot is if I didn't over analyse the details.

One thing I believe is that when two hearts are in the same body like Venra and Terranort, they meld together. The reason you don't see two hearts released from Sora is because they've blended and melded into one heart. I'm not saying this is the answer but it's what I believe on the matter.
Also the reason Sora was able to defeat Ansem SoD in KH1 is most likely because Rikus' body was still fresh and new to him, makin it difficult to intergrate his abilities. Also it's when Sora's heart was at it's strongest, filled with light and connected to all his friends. Oh and it was only a pumped up Heartless, not comparable to a Nobody or Somebody.

 

I'm talking about that second battle with Riku, the one in front of the keyhole. At this point you have an at least mostly possessed Riku. Riku has no way of calling the shots, only a moment of restraint, it is simply his body. I'll concede that as Terranort was a Heartless the two hearts were not complete. However both of their hearts had a lot of darkness, so the heartless were made of most of the heart. Evidenced by the fact that Terranort's heartless is the only one we know that has a human-esque appearance and maintains logic and reasoning.

In BBS Xehanort Report 6 it says there are three kinds of keyblades. Keyblades of Light (Like most of our heroes'), Keyblades of Darkness (Like Mickey's), and Keyblades of Heart. One can make a Keyblade of Heart only by gathering seven hearts of pure light, and this keyblade was the only way to open Kingdom Hearts.

We've seen Kingdom Hearts open twice without it, but we can argue reasons for that in a different thread. Regardless it is a very powerful keyblade and I think it is safe to assume it is still powerful even if it is missing one heart. The keyblade Riku used against Sora in that battle was exactly that. Six of the princesses' hearts made into a keyblade. And it should be noted that it did unlock people’s hearts. It just wasn’t ever shown to be able to open Kingdom Hearts.

Comparing that to Sora who has two hearts (him and Ven) and a normal keyblade of light, the fight seems pretty one sided against us.

Seven hearts were released at that time, one for each princess. Sora's/Ven's heart(s) had no body to return to so they were not shown there since their heart(s) weren't moving anywhere.

 

The composite hearts going into the fight with possessed Riku were mostly weakened; Terra's by being possessed, Xehanort's by being beaten to hell by both the Lingering Sentiment and Aqua, and Riku's by both losing to Sora and losing control of himself. It's not hard to imagine the fully functional (not to mention on a roll) Sora taking the gestalt down. Their darkness versus his light; the fact that Sora won should be enough to tell you that his light was stronger. Besides that, Xehanort's not invincible, though apparently Ventus is: I believe it's been stated that Vanitas is the strongest thing in the KH universe to date, but Ventus beat him. And part of his heart went into Sora.