I'd love to have an entire new map surrounding the doctor's story. Personally, I think that two killers might be too overwhelming for the survivors (players).You might want to keep the killers and their stories separate from each other by adding new maps and a selection screen on the main menu. If you still feel like adding two or even three killers on the same map, you can always add a difficulty setting for the players to choose from. Although, I prefer to have new locations each with their corresponding killer and story behind them.
Thank you for taking time to give your suggestion, indeed you are right that 2 killers on the same area would overwhelm the player. I'm gonna limit the area accessible for each of these killers. The map size is actually doubled at this point and I might separate them into different story as you suggest.