It only makes sense in the sense that Madara did the same thing and it worked for him.
Madara used his Mangekyo till he went completely blind. He implanted his brothers eyes into his own and obtained an Eternal Mangekyo that doesn't go blind. This fixed his own sight even though he was already blind.
Sasuke's situation is mostly the same, except both him AND his brother were basically blind when he finally decided to implant his eyes. But I expect the final result will be the same.
In addition to getting his sight back, I'm expecting him to, at the very least, gain access to his brothers Mangekyo powers.
Sasuke seems to have a better Amaterasu than Itachi(he can reshape it), but Itachi's Tsukiyomi is a much better Genjutsu than Sasuke can perform.
At the very least I expect Sasuke to gain the Tsukiyomi. In addition, his Susasno'o might obtain Itachi's Susano'o's magic artifacts, the Yata Mirror and the sealing sword.
I always assumed it was merging the two together, not replacing them.