I think that when you have brothers and everyone activates their Mangekyou, each brother gains an opposite counter to the other brother. for example itachi had ameratsu, and sasuke had the ability to put out ameratsu. The only reason Sasuke has ameratsu is because his brother transfered it to him by doing the whole poke your head thingie. You also gain the abilities of all the eyes when you kill your brother.
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