Board Thread:Theories and Speculation/@comment-25132842-20180320052324/@comment-25132842-20180320064757

Remember that Hagoromo Otsutsuki could use both the Mangekyo and the Rinnegan. Now, Hagoromo was almost unique in his mastery of the abilities he had. But it does show that there is at least one user who can use both Mangekyo and Rinnegan.

And Madara could also switch between Mangekyo and Rinnegan. I think that most can use one or the other at a time. Sasuke is unique in being able to use both abilities with the same eye.