I honestly just thought that since Xion was a memory being connected to Sora, and having Roxas there at the same time who's also connected to Sora, caused some sort of affect. Riku being who he is...
The thing is Snarl of Memories showed a Memory Sora couldn't have gotten on his own without him having some sort of weird Memory connection with Riku, since it was Riku's encounter with Zexion from Re:Com.
Xion being established on Memories that relate to Sora is exactly why Snarl of Memories holds so much potential - because she saw Riku's Memories.
Don't know much about it either, since I'm not spoiling myself on BBS any more than I have to, but apparently with TAV, it's more complicated than that *shrug*.
They say I've been spoiled about 90% and without even trying. So. :v