Should I separate my finches?

I recently rescued an abandoned male zebra finch and he seemed to be very lonely so I bought another male zebra finch. During the day they get along very well but when one wants to go to sleep it goes in it's nest and then it flies out and attacks the other one. Now my new finch won't nest at all and there are 3 different nests in the cage. Is there any way that I can make them stop fighting or should I just separate them?