okay...lets see...obito with both sharingans means that his kamui will reach perfection...intangibility and long range warping ability is granted..plus he'll gain a third power possibly susanoo. And he has senju dna which means he has the potential to awaken his own set of rinnegan. To be honest, obito doesn't use his left eye rinnegan. How many times have we seen him using it...except controlling the juubi that is...in a 1v1 battle, obito seriously doesn't require rinnegan because he has what it takes to hold his own against any opponent. Even naruto with his speed can't touch him unless kakashi helps him. And when obito has both of his eyes the question of making any contact with him doesn't even arise. Only minato can stand upto him.