Edit: Where are the paths separated? I tried to load and keep going to the end after the dialogue with James, and everything looks the same, whether I choose Nina or Melissa. Is there a way you can start straight with Melissa, without a starting a relationship with Nina and dumping her?
Although I picked Melissa in dialogue with James, I started a relationship with Nina and was able to get only Melissa end-switch or Nina end.
So maybe other decisions are key, probably before the talk with James? In my case, it seemed that any answer to James was insignificant.
Edit 2. In my case, it seems that it was more important if I tried to actively find Melissa at the lake. If so, it opens the new route later, if not, I get the path with Nina. But maybe it is the cumulative effect of more choices during the game. And in some cases the answer in dialogue with James may be the final step that will determine the main path.
Edit 3. Confirmed. With some choices in the past the answer to James is insignificant and Nina's part starts whatever answer you pick, with other choices in the past it is the step after which the routes separate. I've tried several options with quick skipping to get to this conclusion.