When a dragon dies, and the soul is absorbed by a dragonborn, no mere portion survives. The entire soul is devoured by the Dragonborn. If a dragon's soul has been consumed by a Dragonborn (Or theoretically another dragon), it is gone, adding to the power of the devourer and meaning they can not return in any form. The reason the Dragonborn is considered the ultimate dragonslayer by the Blades is due to that fact. If a mortal kills a dragon, Alduin can simply revive the dragon, or if given sufficient time, it can reform itself. However, if another Dragon or Dragonborn steals said soul, the dragon is gone forever. The same principal applies to Dragonborn, they are merely human bodies with the souls of Dragons. Miraak was fully absorbed into the Last Dragonborn after his defeat atop the Summit of Apocrypha, he's deader than dead, never coming back, and his power along with all he stole from other dragons was taken by our hero. I hope this helps.
Also, I hope you will forgive my intrusion into the thread, I'm just quite interested in it and enjoy the Elder Scrolls' lore.