I could be wrong, but the way I took it (and few more of us on IRC did) is this:

It is possible to gain MS through training. Just like Madara and his brother did, but at the same time you can sort of cheat like Itachi did by killing your best friend to gain it. I am assuming Madara didn't know this when he gained it, but that's irrelevent. Also I think that an MS gained Madara's way is stronger than an MS gained Itachi's way.

Now for the Eternal MS you need to have three things: to have gained MS yourself, to trade eyes with someone who has MS (or potential to), and to kill your brother. Obviously your brother has an even greater bond than your best friend, and who knows Kishi may even being throwing in something about genetics there, but I doubt it.

Now one question arising is; does the donor need to have MS or have the potential of MS. I personally believe the latter, but I don't think every Uchiha has the potential which is why Itachi has put Sasuke through all of this, and why he didn't just take one of the sets of eyes of the rest of his family. To gain MS there has to be some form of emotional stress, even if it's obtainable through means other than killing your best friend, but the quickest way is to kill your best friend which is what Itachi was obviously wanting Sasuke to do, but I am assuming after seeing the progress Sasuke has made he knows that Sasuke has the potential to get MS.