I'm going through the same thing with two of my hatchling's right now. I can tell you what others have told me, which I'm in the process of doing also.
1. Put the snake with a small hopper mouse in a brown paper bag. The dark space will make the snake feel more secure, and creates a smaller enviroment. This is thought to entice the snake more.
2. Live HOPPER mice are what you want to be feeding in the first place. Pinks, and fuzzies are good to assist feed with, but don't make a lot of movement. They might be looking for a more feisty meal.
Like I said I'm still in the process of trying these myself, but its good advice from knowledgeable people on here. Good luck and don't give up!
EDIT! Separate them ASAP! That is most likely your culprit right there!