He'll probably 'borrow' both of Obito's eyes, since he isn't exactly going to be using them anymore. He'll borrow the other sharingan only because he needs to get to Nagato's corpse in order to retrieve the other one.
Flashback to Zetsu already getting that one.
It makes perfect sense what is going on. He was brought back without his eyes cause of what Kage pointed out... So naturally he steals some Senjutsu chakra from the First to enter sage mode because of the sensory abilities it grants the person. Really he probably has great sensory skills then the Sharingan right now.
Edit in: also if you recall Kabuto made a remark to his edo zombie self having rinnegan was a gift. And if we look at it another way, it would make no sense for something that is exactly the same to coexist at the same time. Madara's living eyes are already in the world.
Also, why didn't Sasuke just light his ass up with some raiton currents through his katana? There is no sign that he can absorb ninjutsu that fast, black flames burn slow I believe. Hell there is no sign he can flat out absorb ninjutsu anyways, I'm sure he can co trol those flames like Sasuke perhaps even without Sharingan. He has a high level of Katon usage after all.