a lot of people say that in the databooks, it states you need both amaterasu and tsukuyomi to use susanoo but i think this is incorrect. you don't need both, you just need the power of two MS sharingans. take a look.
he says 'along with' meaning that susanoo was the third techinque after the first two, he does not however say that amaterasu and tsukuyomi are needed.
Sasuke says that only those who can control the double mangekyou can use susanoo meaning that only those who have mastered their MS abilities can obtain susanoo.
when obito says a 'a sharingan that can activate even susanoo', i think he is referring to the fact that not everyone who had MS could master it.
lets take a look at the people who we knew had MS.
Madara: has used susanoo but has yet to use tsukuyomi or amaterasu
Izuna: if madara had it ima guess and say he had it too
Itachi: was a genius even for an uchiha and able to master MS to use susanoo
Sasuke: like it or not sasuke was also talented and was able to use susanoo
Obito: his potential is unknown as he only has 1 of his eyes and as stated by madara, both eyes are needed to use the full power of the sharingan. he may have been able to use susanoo if he had both. i believe that this is part of kishi plot to weaken obito and make madara stronger. as obito also had senju dna i believe he would also have been able to use PS. he would have had 2 teachniques in kamui and the S/T technique (which might also be called kamui but they are different powers)
Shisui: now he is the confusing 1 because although he had both eyes, as far as we know he had the same technique in both eyes. it could be possible that he had another technique and did not master it like he did kotoamatsukami. it could be that he did master his other technique and could use susanoo but we never got to see it. only kishi knows lol.
i think the only reason why both itachi and sasuke use tsukuyomi and amaterasu has something to do with the fact that they are brothers or because itachi gave sasuke his power.
i am not saying that i am 100% correct but hopefully kishi will give us some answers, through madara.
right now, the answer rests on whether or not madara can use tsukuyomi or amaterasu, until he does then i think my theory is correct