Technically speaking, neither. See my post #37, starting at the sentence, "Humans have 23 pairs of chromosomes"...
Chromosomes tend to undergo fission and fusion, but the genetic material doesn't magically "appear" or get "lost", it just gets repackaged.
Your post says " even a bit of lost overlap." When you can repackage the chromosomes in a lab and create a new species, or prove this by showing the world a current day mutation creating a permanent species, then I'll believe it.
Until then, you are just speculating.