What's the difference between "Inzwischen" and "Mittlerweile"?
"Mittlerweile" is not as frequently used as "inzwischen". In some situations, they are interchangeable, but not always. "Mittlerweile" expresses a relation to the point of time at which the sentence is said, whereas "inzwischen" can refer to instances in the past or in the future.
(This is just my summary at the moment; it's always difficult to explain individual words "in general".)
Well, "Inzwischen" means "meantime", and "Mittlerweile" means "now". Some words in German mean the exact same thing but are used in different words. That is also English. For example, "happy" means the same as "joyful". They both mean the same thing, but are different words. This is just my opinion, and I may be wrong, but that is what it should be.