Board Thread:Naruto Discussions/@comment-34436445-20180703141210/@comment-3273613-20180731145135

It has been said by Madara that having Rinnegan in both eyes makes the Rinnegan powerset more potent.

However, the Rinnegan is not bound to the powerset rules of the Sharingan/Mangekyo, wherein a single eye grants access to a specific power, like Obito's Kamui, Tsukuyomi, etc. The Rinnegan only enhances the power but one eye nevertheless grants you all techniques.

As for the tomoe, it is irrelevant. Purely aesthetics from the author and storytelling device for the audience to quickly understand how much power Sasuke has left to recharge (it loses all tomoe when he spams eye techniques and returns after a while)

For the topic itself, I think that is due to being half of Hagoromo's power and so he activated a single, but unique style of Rinnegan.