If I remember correctly, assessment tasks that require the use of a related text alongside a prescribed text prohibit the use of another prescribed text as a related text. However, this applies to prescribed texts for the subject itself. For example, the prescribed texts for English Advanced do not extend beyond those listed under English Advanced, meaning that prescribed texts listed under other levels of English are technically treated the same way as any text outside the syllabus (unless they are common between both English Advanced and English Extension 1 in your case), and may be used as a related text. The same may apply to English Extension 1, that is, prescribed texts for English Advanced that are not also prescribed texts for English Extension 1 are essentially foreign, i.e. the same as related texts not mentioned under the list of prescribed texts for any level of English.
With that being said, your ability to use a prescribed text for English Advanced as a related text for English Extension 1 may be impacted by a condition set as part of an assessment task description at your school, and/or an underlying requirement of HSC English. As such, I would recommend checking your assessment task description (if applicable) for any such indication and discussing this aspect with your teacher for further confirmation if needed.
I hope this helps!
