It's not only both MS eyes, that's just one of the requirements, the other is to have the Yin affinity to use it, something only incarnates and people of similar level possess.Indra is an ancestor but it's still an MS ability. You see MS users themselves are rare let alone susanoo which requires both MS eyes. And then taking to make a perfect Susanoo. Indra is also the ancestor of the Uchiha he embodies the strongest eyes. His descendants however share his power whether they're incarnates or not. Dome bring it to fruition and the others do not.
Itachi had Susanoo because his Yin was strong enough to rival MS Sasuke's and grant EMS, it's why his genjutsu was so haxx, illusions are highly dependent on mental energy which is Yin.
Sasuke and Madara had Susanoo because they had Indra's Yin which is the strongest, Obito's chakra gave Kakashi Susanoo because Sage chakra gave his dojutsu similar levels of Yin as Indra (Indra had MS powered by Sage chakra)
Having double MS does not necessary equate to having the Yin affinity to use avatars.