In the 2001 HSC more than one related text was required:
In your answer, you should refer to your prescribed text, ONE text from the prescribed stimulus booklet Changing, and other related texts of your own choosing.
"Other related texts" (note the plural) means you have to do more than one related text. So it is certainly possible that you could be required to write about more than one related text. For me, I've got three related texts (I must be crazy!!)ready as backup, though two of them are films. I initially plan to go for one related text, but if I have more time I'll go for a second related text.
If you really want a strong argument you should use a second related text. A lot of the high range responses in that Student Answers book and the exemplars use more than one related text because they have an extensive argument and extensive textual support. However, if you're running out of ideas to support your argument, it's probably better to do one related text.