A Nobody is composed of a person's body and soul, therefore, it's impossible to have two nobodies.
Ok, Sora still has his body as Roxas is running around. Roxas was created when Sora released Kairi's heart and Sora still has his body, which means there are two bodies now. Same thing with Namine and Kairi...whose bodies actually meet each other. I understand that they are special cases, but Xion is also part of the Kairi/Sora/Roxas equation, so it doesn't seem impossible to have another nobody born of another truckload of special circumstances. I personally don't think that there ARE two nobodies for anyone, and that either xion or namine is an unbirth. But the ONE body thing seems pretty much fucked.