Rat helpplease
http://www.ratsociety.org/quarantine.htm...
Are you sure they are fighting? Is there blood being drawn? Is there shrieking coming from the cage when they are together? Is the new boy cowering in fear? Are you seeing any of the behaviors, as listed here, in your rats?
http://www.ratbehavior.org/aggression.ht...
How old is your boy that you have had for 2 weeks?
How old is the new baby?
It is very odd for rats under 12 weeks old to be fighting already. It could be that the older boy is telling the younger boy who the boss is with some rough-housing and power-grooming. This does look like fighting if you don't understand rat behavior. This is normal, but it sends new rat-keepers on edge. The following will tell you how rats play together.
http://www.ratbehavior.org/ratplay.htm...
Did both the boys come from the same pet store? If not there needs to be a quarantine period for the health of both boys.
http://www.rmca.org/articles/quarantine.
Were they, at one time, in the same cage together, or are they from the same store but from two separate cages?
Ok, let's start over. For now keep each boy in a separate cage, and put the new boy in another room. Each has to calm down, get use to their surroundings, and get use to you.
E-mail me spazrats@yahoo.ca in 3 days and together we will start the introductions that will bond the boys for life. This can work, it will just take longer then you expected. In this period of 3 days take a handful of bedding from each cage and switch the bedding into the other's cage.