No, you are never required to study more than one prescribed text, one stimulus booklet text and possibly two related texts. However, in recent years they have only asked for one related text, because it is difficult to write about four texts in 40 minutes, in a substantial manner. I spoke to my teacher and she said that the Board of Studies stated that they will never ask for more than one Stimulus Booklet text. I hope that helps.
What I do recommend though, if you still are unsure is that you have a Stimulus text that you know very well, and a backup, just in case your second text works better for the question. Just make sure that you can link both well to your area of study, i.e. Physical, Inner or Imaginative Journey.
Good luck, hope that helps.
Nessa...