How does Obito have two Sharingans if Madara took one? If Madara took Obito's left eye, how come he has both Sharingans? (Chapter 683, page 15) Did Obito replace the one he lost with Kakashi's Sharingan, like a trade?
TL;DR:
Because Madara gave him the Sharingan he had taken from Kakashi back when he took the Rinnegan Obito had in his left eye socket.

Long version:
In chapter 674, page 14, Madara takes Kakashi's Sharingan eye (Obito's left eye):  



In chapter 675, page 10, Madara switches his left eye - currently holding Obito's left eye - for the Rinnegan Obito has in his left eye socket:



In that same chapter, he lets Black Zetsu take control of Obito's left side. In chapter 676, page 2, we see that Obito now has both of his Sharingan, though he is not in control of his body:



In chapter 679, page 6, Black Zetsu leaves Obito's body to join Kaguya:



Leaving him finally in control of both of his Sharingan when Naruto revives him and he regains consciousness in chapter 683, page 6: