Over time, use of the Mangekyō Sharingan's techniques deteriorates the user's eyesight to the point of blindness. Only by taking and transplanting the Mangekyō of another Uchiha — integrating the two pairs of eyes in the process — can the vision be restored, a process that results in the creation of an Eternal Mangekyō Sharingan (永遠の万華鏡写輪眼, Eien no Mangekyō Sharingan). It would appear that the tighter the blood ties are between the "donor" and the "receiver", the better it is for compatibility — a sibling for example seems to make the best donor. The time from fusing the donor's Mangekyō with the receiver's Mangekyō to the point of full recovery is at least several days as seen in Sasuke's case.Not only does this process permanently restore one's eyesight, but it also gives the person even greater increases in power. The Eternal Mangekyō Sharingan also changes form from the original Mangekyō form, to what appears to be a fusion of the wielder's own Mangekyō with the transplanted one.
so Basically its a fusion of both Eyes, as it was also seen hinted during the course of the series, in Itachi's Story with madara getting EMS you saw a Demon behind him with Four eyes representing his Eternal Mangekyo Sharingan and when Itachi put Sasuke in a genjutsu the same Demon appeared behind Itachi but with only TWO eyes reprsenting the Mangekyo Sharingan and the other Two Sockets where empty....and in Recent Chapters Sasuke still has access to his original Mangekyo Sharingan showing he can Switch it to Eternal Mangekyo at will
