Slick, Carlisle slick.

The Sharingan is a devolved form of the Sharinnegan--Hagoromo inherited the Rinnegan eyes from Kaguya's Sharinnegan and Indra inherit the Sharingan from Hagoromo along with the spiritual half of Hagoromo's chakra, the physical half--vitality, body and all that went to Ashura. You need both Indra and Ashura's charka specifically to recreate the Rinnegan--the combination of spirit and body. Tobi/Obito isn't the reincarnation of Indra nor Ashura. He is descended from Indra--as are all Uchiha, has Madara's Rinnegan and Hashirama Cell's and still literally states that he is too weak to withstand a second rinnegan. Nagato had a strong enough body and charka because of being an Uzumaki even though he wasn't Ashura's reincarnation--but his body isn't designed for the Rinnegan or doujutsu and thus it drains him incredibly--yet, he still can use two and surive unlike Obito.

The Rinnegan isn't 'Uchiha' lineage, it's the combination of Senju/Uzumaki and Uchiha. It comes from Hagoromo whose power was split into two between his two children, who went on to create the Uchiha(Indra) and the Senju/Uzumaki(Ashura). Obito had the eyes--his body was designed as it were--for doujutsu, yet he lacked the strong body and powerful chakra of the Senju/Uzumaki and thus couldn't handle more than a single rinnegan--he partically states as much when he puts in one rinnegan and describes how it feels and how he knows as he is currently even with White Zetsu(Hashirama Cells) making up a decent part of his body he wouldn't be capable of withstanding a second rinnegan. Nagato lacked the Uchiha lineage but as an Uzumaki his body and charka was strong enough to withstand two despite those eyes not belonging to him.