The only way to obtain the Eternal Mangekyou Sharingan is by obtaining your "siblings" Mangekyou Sharingan, while you have a Mangekyou Sharingan.
The reasoning of this is due to blood infection. Yes, I know you will probably try and throw that "But Kakashi obtained Obito's sharingan". The thing about that is he was lucky as heck that Obito had the same blood type as Kakashi. But since he wasn't an Uchiha the Sharingan eye is always active and takes the double amount chakra. But that is a different story.
Say your an Uchiha, and your trying to obtain the EMS by obtaining your friends MS. But the problem will end up being are the blood types the same because if they are not you have a high chance of dieing or not being able to use that eye socket ever again.
Thats why they made it you have to have your "siblings" MS to obtain EMS.