at my work we would have several males living together peacfully. then one day someone decided to split them up. I put them back together and they fought badly. causing blood shed even. How old is the gerbil? Sometimes males wont accept another male into its territory if it has been alone for a while.
Even if they are brothers.

I would watch them VERY carefully. if they continue to fight even after slowly introducing them, i would seperate them .

good luck