dont just rely on me but i would say it is not, you could still acheive a top band result using 2 prescribed texts (as well as related) assuming that you are able to show using these 2 texts that you fully understand the concepts inherent in your module. In other words if you can say all you think you need to about what you want to using just 2 texts then you will be fine, but if a key element of your module is featured in one of the texts and it is neglected then you could be in trouble. My advice would be to know 3 but only do 2 (but really well and in depth) which 2 depends on the demands and constraints of the question. 2 texts done well will get you closer to a top band then 3 or 4 sketchy "only scratching the surface" paragraphs on 3 different texts. I would suggest also though that the 2 texts you do use are different in nature perhaps even contrasting each other so that you may present a equally weighted discussion of your focus. As i said dont just take my advice, but that is what i am doing.