Actually I didn't get that part with double MS... xD
Well the left MS and right MS each used seperately gives diff techs.. Tsukyomi n Amaterasu.. we all knw that.. so may be for someone to use MS in both eyes at same time can give you diff powers.. may b thats wat he means when he says "those who can control double MS (I guess MS in both eyes) at same time can use Susano"..
I made a thread a while back when Sasuke used MS in both his eyes to extinguish amaterasu during his fight with 8-tails..
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May that was when he realised it.. caz he said to his team after the fight that he doesn't need any tailed beasts power as he has gained a new power.. so may be at that time he figured out how to activate Susano or how its powers work.. by activating MS in both eyes..
Also this chapter page 07.. I think Sleepyfans made a typo... they shud have said his
Right Eye and not
Left eye through wich he is able to manipulate black flames... left eye activates the flames n with right eye he manipulates them as was said in the spoilers and also it was evident in this chapter when he used the "ento kagetsuchi" by manipulating it with his right eye.. pg 02!!