Hi,
I've spoken to a few teachers about these structural issues, including a couple of senior markers for adv and Ext I. What I've essentially been told is that the core text is far more critical to the success of your response.
The primary reason for this is that the core text is the base for comparison with other students. In the limited amount of time you have, you can only conjure so much original and creative insight into the texts. Given the fact that the core text is the text that you study in class, and the text that there are likely more resources for (such as journal articles), it's probably far more likely that you'll be able to produce better analysis and insight into the text. Thus, you'll want to put your best foot forward. Placing the core first will allow for the marker to more easily distinguish the (hopefully stronger) analysis/essay, as they'll have a much stronger frame of reference for the quality of your work. This is important given the fact that some external markers spend 2 minutes marking an essay.
Ultimately, the purpose of your related text is to strengthen your thesis by providing a counterpoint and/or additional evidence. You can easily get away with having 60% core and 40% RT in an essay, but not the other way round. Generally I personally want my core to be the ground from which my thesis is evolved, with the RT acting to add nuance and a different perspective but concurring response.
Of course, English teachers love to disagree, and there's always an exception to the rule, just remember that its called the "core" text for a reason.