Re: I have an idea ...
of course madara=tobi. ignore his post.
n1 idea, but i think that if 2x ms would have the same result as ems even if you already have one.
however, sasuke got the ms-abilitys of itachi so it's the question weather the amaterasu he later used was his own or itachi's. it is possible that he has his 3 own, yet undevelopped/unshown ms-techniques.
The result of that would b that itachi's ms-techs were somehow "learned" techniques linked with the kg and therefore not abilitys of his eyes. madara wouldn't get them if he killed sasuke and got his eyes, neither with itachi's ones, cause itachi didn't had them anymore when he died. just if sasuke would get the ems itachi's abilitys were linked to his eyes again.
however, i don't think that madara wants to get the ms of sasuke, it's simply not necessary. weather he has got 3 or 4, madara is nearly unbeatable.