Naruto had the power to regrow body parts of any individual as we saw with Kakashi. Heck, he restored Guy who was on the brink of death. Therefore, was it not possible for Naruto to take Obito's left eye, transplant it to Kakashi and then just restore Obito's lost eye? It would have been the perfect scenario with Kakashi having a new sharingan and Obito having his both eyes and being able to access the full power of Sharingan.
The Sharingan and Mangekyou Sharingan are caused by a special kind of ocular chakra originating in an Uchiha's brain. Usually, a cause of powerful emotion will awaken the Sharingan, and great loss will evolve it into the Mangekyou. Once unlocked, activating them is fairly easy. They can be transplanted, and even retain their abilities. Though it may have been an exception due to its spacetime powers, Kakashi's case showed it could even evolve to the Mangekyou despite not being in an Uchiha. Kakashi could activate and deactivate the Mangekyou form. However, he could never deactivate the Sharingan, and hence had to cover up his Sharingan eye when not in use to preserve chakra.
Next, after Obitos's death, he used his space time powers to transfer his chakra to Kakashi. By doing so, he also transfered his ocular chakra, which temporarily awakened Obito's Mangekyou in Kakashi. Once that Chakra was used up and the Sharingan released, it was gone for good.
Lastly, there is one who went through a very similar situation, Madara. As pointed out, Madara at one time had Obito's Mangekyou and his own Rinnegan. By having a 2nd eye, despite having unlocked the Rinnegan, did not evolve it into a Rinnegan. He even transplanted regular Sharingan before his death, but they did not evolve into Rinnegan either, despite up to several years between him giving his Rinnegan to Nagato, transplanting the spare Sharingan and dying.
Now, of course those were not Madara's real eyes, but the fact that he had the ocular chakra required to awaken even the Rinnegan, but couldn't do so with a Sharingan and a Mangekyou eye says a lot.
With all that established, We have a decent amount of evidence backing up what would happen if Naruto grew a new eye for Obito. Most likely, what would happen is the new eye would be a normal Uchiha eye, which is almost identical to any other regular eye. It would have the potential to unlock the Sharingan and Mangekyou Sharingan, and Obito could probably more easily unlock them than normal, but its impossible to say if it would happen immediately, take months to do so, or anywhere in between.
So, transplanting Obito's left eye into Kakashi, and then having Naruto regrow Obitos eye would likely be able to create 2 eyes with Obito's Mangekyou powers, given an unknown amount of time before he could reawaken his powers.
That however shows the main reason why they didn't do so, besides the situation preventing them from thinking about it. They couldn't risk it not working instantly. Obito was under Zetsus control until Kaguya showed up, and they needed everything they had to just barely pull off that victory. Obito having his left eye's powers is what both got Sasuke out of the dimension Kaguya threw him into alone, and saved Kakashi from her All-Killing Ash Bones. The odds that his new left eye being fully awakened to Mangekyou within the few minutes before it was needed are slim to none. Then after it was used twice in critical circumstances, he turned to ash.