Let's say Uchiha A and an Uchiha B both have awakened their Mangekyou Sharingan. If A and B exchange their eyes, can they unlock Eternal Mangekyou Sharingan?
In the Manga, only:
- Uchiha Madara,
- Uchiha Izuna (Madara's brother),
- Uchiha Shinsui,
- Uchiha Itachi(Sasuke's brother),
- Uchiha Sasuke,
- Uchiha Fugaku(Itachi's and Sasuke's father)
- Uchiha Obito,
Is shown to have awakened Mangekyou Sharingan (henceforth the MS). Of those 7 only 2 is confirmed to have an Eternal Mangekyou Sharingan (henceforth the EMS), that is Uchiha Madara and Uchiha Sasuke.
Madara gained his EMS after taking his brother's MS. Note that he claimed that his brother gave it to him. Madara did not take it from him by force. When Madara told this story, his brother was shown to be on his deathbed. By the time Madara gained his EMS, his original eyes has lost its light, he is practically blind.
Sasuke gained his EMS from Uchiha Itachi. At first he hesitated in transplanting Itachi's eyes into his. After his eyes got worse from the extensive use of MS techniques, he agreed to the transplantation and thus gained his EMS. Itachi's eyes were also near blind.
There are 4 points that is similar between the two.
- One is that they have their original eyes evolved into MS.
- Two, they transplanted the other eyes from their brother.
- Three they transplanted their brother's eyes when their original eyes are blind or near blind.
- Four both's brothers are dead when they transplanted the eyes.
IIRC it was told in the manga that transplantation is better done between close relative since they are more compatible.
If, only the first two conditions are necessary for the EMS then yes, any close relative that has awakened MS can simply exchange eyes for both to awaken EMS. If point 3 is also need to be met then they still can exchange eyes after exhausting their eyes, making it near blind. If point 4 is also necessary then they can't simply exchange eyes as one needs to die before the other can transplant their eyes and awaken EMS.
In order to activate the EMS you would need the eyes of a relative who has activated MS the prime example of this is Obito he transferred plenty of other sharingans into his missing eye socket as there was more than just him and Itachi with MS during the time of the Uchiha Massacre (where he took the eyes of uchiha that have activated at least a regular sharingan) so its safe to safe hes at least transplanted one other eye with MS in that eye socket but has never awakend EMS.