I've owned rats before. The rat in the picture is likely young. Sometimes, young rats and adult rats don't get along well. I think its a dominance issue. They tend to do that if they were not raised together.
I recommend trying to keep them separate until the young one is an adult or at least bigger. Chances are then they might be able to get along.
And this is assuming that they're both female!! You can guess what happens if you mix genders...
Also, I second ChoclateBetta- a different habitat would be nice (plus that rat could jump/chew its way out of the box). I suggest giving them something to play with. :) Rats are social, intelligent, and playful. They love toys. I gave mine an old sock to play in, chew up, and make a nice little nest. They love it.
Hope that helps some. Also, some links? The first is a good website for rat owners. The second should hopefully help with the fighting rats.