Naminé?

[JTD95]

A nobody can't exist at the same time as their somebody, right? So how can Naminé exist at the same time as Kairi?

 

[Vice]

Special Nobody.

Also Roxas and Sora Existed at the same time.

 

[Smile]

Special Nobody.

Also Roxas and Sora Existed at the same time.

You can't really compare it, seeing how Roxas had Sora's B+S, while Sora was just a Heart.
Kairi, however, was a Whole Being, despite Naminé being born.
Lacking a Body and Soul to call her own (Roxas has Sora's, and Kairi kept her own all through KH1 and just needed to have her Heart put back in), there's nothing holding Naminé back from being around while Kairi is still effectively a Whole Being made up of her own B+S+H.
She's special, yes, but in a different way from Roxas, one that lets her Somebody be... well, a Somebody in the sense of a Whole Being, while she's still around. She never took anything from anyone, unlike Roxas.