If the EMS is a combination of two Uchiha's Sharingan and you get the ability of the other user as well what the hell happened with Madara? With his own MS he was a match for Hashirama and he got his brother's eyes and his brother was Madara's equal so shouldn't Madara have been more than enough...
This is probably stupid and all but I'm not sure if someone already thought of this but since Madara gave Nagato his Rinnegan therefor giving Nagato Uchiha DNA as well as his own Senju DNA would it have been possible for his Rinnegan to digress to the Sharingan? And Why would it have been stuck...