Meh... guess I'll explain my reasoning so you don't think I'm a total fool Muffinman.
My guess is that Riku Replica became a Non-Existent One just like the members of the Organization. He faded into nothing when he was finally defeated, but the probably made him the perfect subject for being a member of the Order. After all, they probably weren't officially called the 13th Order until somewhere during the one year time period while Sora is asleep since KH2 is supposed to take place a year after the events of KH and CoM. I could get even MORE technical but I'd really rather not. But I'd have to admit it be pretty cool if we got battling Riku twins in KH2. ;D Eh, could have this all horribly wrong though since we don't know who the remaing Unknown/'s are/is.
In conclusion... Sora is NOT the Dual-Wielding Unknown, and I don't believe BHK is either.