Board Thread:Theories and Speculation/@comment-1512913-20150827121154/@comment-734582-20150830132630

According to Itachi not every Mangekyou can become an Eternal when transplanted, it has to be close family. It's likely that Itachi's eyes were flooded with Sasukes chakra, creating the EMS (So the eyes themselves have a merger of this chakra). It's not clear WHY this happens, but it's likely that the Uchiha bloodline has become too weak to sustain a proper Mangekyou. By obtaining an eye with a merger of sibling chakra, the eye may have enough strength to withstand it's own power. After all, Obito had Hagoromo's cells that strengthen him, and his eye never grew weak.

Hence, the MS light tradeoff is essentially an "impurity" that transplanting sibling eyes fixes, even when the transplanted eye had already lost all light (Itachi and Sasuke both had lost a lot of light, Itachi's eyes even entirely - yet the EMS had eyesight fully restored). However, if the eyes are not related enough the EMS may not be attained.

As to ownership, i believe the EMS is a perfect merger of eyes, even though it only has the power of the receiver. As a result, i think that either Sasuke or Itachi would be able to use their EMS with equal skill and ability.

As to ownership of the Rinnegan, the Rinnegan requires the user to have the body of a Senju just to be able to use it (Nagato/Obito could use Madara's rinnegan, but neither to it's full extent). Given my above idea of the EMS, it's likely that if Izuna or Itachi achieved the same body as Madara/Sasuke, they could control the Rinnegan as if it was theirs. A condition would be that their body reaches their natural state for their own Rinnegan. After all, Madara did not awaken it until he was near death. Izuna would not be able to truly call the Rinnegan his until he reaches a similar point. At that point, the Rinnegan should be able to be freely shared between the two.