Board Thread:Versus Debates/@comment-29899821-20160317090255/@comment-3965545-20160512222422

Seelentau confirmed that those with Double Mangekyo usually awaken Susanoo upon mastering usage of both eyes techs or something like that. So Shisui having Susanoo is not a stretch or implausible. Obito did state that Susanoo was a rarity among Mangekyo users regardless but that was earlier in the manga and it seems DB4 and the later chapters have retconned that seeing as how Kakashi instantly used Susanoo once he got both Mangekyo. So all it takes is both Mangekyo and both techs knowledge of and usage, therefor Shisui most definitely has Susanoo.