You might be able to get away with one, but if you learn two related texts, you have a back up for the exam in case the first one doesn't suit well. Also, including two in your essay shows a greater depth of analysis and allows you to contrast and compare better.
My teachers have told us that you should know 2 ONLY because they CAN ask for 2, since the rubric says "Related texts". They said if they give you the option though, to explore only 1, because the ideas will be more logical and cohesive throughout.