Obviously the Allies didn't expect to have East and West Germany, so it would't have been a problem to have Berlin occupied partially by the Allies, but why did the Western powers want to have occupation zones in Berlin anyway?
The entire of Germany was originally split up into 4 zones, each controlled by GB, the USA, the Soviet Union and France. The 3 zones controlled by the Western powers were then amalgamated into one state with a single currency, so originally the entire of Germany was divided in the same way that Berlin was.
Berlin however was, and is, the political and cultural capital of Germany and as such was deemed such an important city that despite its location (Deep in the Russian Zone of Germany) it too should be split into 4 parts in order that the most important city in Germany would not be controlled entirely by one power.