Do NOT have the first half of your essay discussing one of your two case studies and the other half discussing the other case study. Structure your essay into paragraphs that directly examine either similarities or differences (obviously you need to look at both).
Do this instead of just thinking about what information you will include.
Make sure that when you spend the first few minutes of the exam planning your answer, you decide on what are the key similarities (compare) and the key differences (contrast) that you will structure your answer around.