Okay I just called up the Board of Studies (unhelpful son of a bitches)
To be honest, the guy was pretty patient since I kept asking virtually the same question over and over.
This is how it went:
Me: So how is section 3 going to be marked for the people that wrote 2 related texts?
Dude: The marking centre will be briefed on the situation and the papers will be marked accordingly.
Me: ..What? So what does that actually mean?
Dude: Well this matter is up to the marking centre, but what I can say is that you won't be directly penalised for using 2 related texts (goes on to explain positive marking). The responses will be marked according to their quality.
Me: Okay so people that used 2 won't be penalised, but are they still going to be marked normally? (Do you see what I mean by me asking the same question over and over)
Dude: The marking centre is going to have a briefing since lots of students wrote one related text and lots wrote two. Then depending on the quality of the response, they will mark the responses accordingly.
Me: ..Wait so if it's all the same marking as always, isn't that unfair for people that wrote one related?
Dude: Did you write two or one?
Me: One.
Dude: Okay, in that case your response will be much more in depth then people with one related text and that will be taken into consideration when being marked.
Me: Riiiiight..
Dude: Would you like to speak to someone else?
Me: Nokthxbai
TL;DR they're going to have a 'formal briefing' with the markers as to how to handle the situation (we don't know what's going to be said at the briefing). Both related texts will be read and taken into consideration. Since the question only requires one related text, generally speaking people that used one will have better quality/greater in-depth responses (due to the amount you can write in 40 minutes) and hence, will be marked higher than people that wrote about two (because you're giving less quality and unnecessary quantity). So if you wrote two but you wrote really damn good then it will get 15/15, it's just harder because people that wrote one have more analysis on each text.
TL;DR you won't be penalised directly, only in that your responses may be weaker in quality than people that wrote one related text
TL;DR relax