Just because her brain wasn't there to add to the Tuvix personality... Are we sure that her Ocampan DNA didn't effect Tuvix's brain construction? Although some of her personality cold have slipped in just from "backups" in something we don't understand which is why clones from fingernail cells in sciencefiction always seem to have a complete set of memories from the donor...
Tuvix was really Keelok.
The Kes component in the mix sabotaged the entire existence of this beings life.
Rather than bing forced to frakk herself, Kess inside Tuvix made sure that Kes outside Tuvix didn't want Tuvix inside Kes.
In Fury, how hard from a special effects budget point of view would it have been for old Kes to make out with young Kes?
It couldn't have been more disturbing than what i saw on American Horror last night.