When I was doing C# code reviews at first in VS 2012 I was getting a side by side comparison of the old and new code. However now I am getting all code in the same window with r
Below screenshot will help in setting the option to compare side by side.
There should be a button on the toolbar like this one that will let you select the 4 different "diff modes".

You can also use the following keyboard shortcuts:
Ctrl+\, Ctrl 1 - InlineCtrl+\, Ctrl 2 - Side-by-sideCtrl+\, Ctrl 3 - Left OnlyCtrl+\, Ctrl 4 - Right Onlyto change the view.