one plus one is still 2

[Dust]

So Madara explained why Tobito is able to use Kumai at such an advanced level. He explains to Tobito that one needs two sharigan to reach full potential. Kakashi never had a chance. (We see that Tobito had an implanted Sharigan in his left eye socket when he used Izanagi against Konan.) Then we learn that half him is "Hashirama artificial body" which helps inhance his optical powers ( senju+Uchiha). Finally he implanted the original eyes of Madara from Nagato which we know are mutated sharigans so he still possesses 2. It's funny because in chapter 602, Madara is already teaching OBito how to be stronger. he even suggested having a stockpile of sharigan which we know that tobito has:noc:

 

[Silent Zetsu]

So basically. Obito got stronger by collecting sharingans and using senju DNA