Why the hell would the Dark Wanker's active soul fragment go to King's Cross with a dead Harry and a dead Dumbledore? Voldie was still using it. The fragment in King's Cross is the fragment that was in Harry, that was killed by Voldie's AK.
Because when he hit Harry with that spell, he damn near killed himself, too. It's explicitely stated that when you destroy a Horcrux, you destroy the soul contained within. The Horcrux in Harry was done, finito, gone; it wouldn't have been at King's Cross with him.
Basically then all of the fragments of Voldemort's soul except for the one in Nagini should have been there.
I get the feeling you really just don't want to like this book or you skimmed it. I recommend going back and reading it. The Horcruxes were destroyed, and with them the fragments of Voldemort's soul were destroyed utterly. If they merely moved onto the afterlife indivudally, there would be no consequence for him, as they'd be together again once all the horcruxes were destroyed.
Am I the only one who read Prisoner of Azkaban, Order of the Phoenix and Half-Blood Prince? The details are all in there.
PoA: The reason the Dementor's Kiss is so feared and horrible is that they suck out your soul. It's referred to it as "a fate worse than death."
OoTP: When battling Voldemort, Dumbledore tells him that there are fates worse than death. Voldemort scoffs at the idea.
HBP: Harry discovers what a Horcrux is from Slughorn and that Riddle had sought him with inquiries on them. He learns that you contain a piece of your soul in it and Dumbledore reveals to him that if said Horcrux is destroyed, so is the piece of soul inside.
DH: Harry destroys all of Voldemort's Horcruxes with his friends' help, thus destroying Voldemort's Soul, essentially. A fate worse than death when we take the simple logic introduced throughout the prior books.
Don't overthink it, it's a simple line of retionale introduced for a children's book. Overthinking it just ruins it. Not everything in life and literature has to be a complex series of riddles.