What does it mean when my birds fight a lot? what can i do about it?

well i have 2 zebra finches and i had to separate them recently because they kept attacking each other and i thought they were going to kill each other so i separated them. i don't know why dis could b happening n i would like 2 noe wat i could do about it? do i put them back together??? help!!!! ooh n they hav a nest n i think she is breeding she's inside they nest a lot. n again help!!!!