Board Thread:Theories and Speculation/@comment-1512913-20150221101637/@comment-6769189-20150221194122

The Rinnegan may not be necessary to get the Rinne Sharingan. Maybe if someone was strong enough with the Rinnegan they could obtain it from the 10 Tails. As I recall Obito was too weak for 2 Rinnegan, so by default too weak for Rinne Sharingan.

To my understanding Kaguya obtained it through the Shinju, so she started with it.

The Rinne Sharingan seems to be Rinnegan+Sharingan. So the Sharingan provides the Tsukuyomi while the Rinnegan provides the Infinite.
 * Rinnegan has access to Sharingan techniques, because its the same eye as a Sharingan(if one is an Uchiha), But the Rinne Sharingan is what happens when they truly merge.

I always believed the Sharinnegan to be the middle stage between Rinnegan and Rinne Sharingan, being good enough to dispel the IT.
 * So the the Rinne Sharingan devolved into the Rinnegan, which devolved into the Mangekyo and Sharingan. the Sharinnegan comes between Rinnegan and MS/Sharingan.