Conversely, you will notice the modules which require the use of related texts are 'topics', eg. Belonging or History&Memory rather than just critical analysis or comparative study.
So in using more than 1 related text, you can explore the topic from multiple viewpoints to give you greater understanding of say, belonging, which is what will give you the depth in your essay.
If you are good enough at english, you should be able to sufficiently analyse 2 related texts in enough depth. Or if you can write fast, you can get a few hundred words over the average student.
But anyway, its best to have 2 prepared, but know one of them in greater depth than the other.