Simple theory that just popped up in my mind. It's probably flawed in some way, but that's why I leave you readers to determine it.
I propose that when Sora & Kairi's Nobodies were created, they may have had a pair created in both the Realm of Light and the Realm of Darkness to correspond with the location of Sora & Kairi's hearts during the creation process (Sora was fading into darkness, whereas Kairi was reawakening in the light).
Namine and Roxas may be their Nobodies from the Light Realm.
Whereas Xion and the friend she's searching for are their Nobodies from the Dark Realm.
I propose that when Sora & Kairi's Nobodies were created, they may have had a pair created in both the Realm of Light and the Realm of Darkness to correspond with the location of Sora & Kairi's hearts during the creation process (Sora was fading into darkness, whereas Kairi was reawakening in the light).
Namine and Roxas may be their Nobodies from the Light Realm.
Whereas Xion and the friend she's searching for are their Nobodies from the Dark Realm.