It sounds like they are being a bit hormonal. I would give them 12 hours or dark it may help, here is an article on hormonal tiels http://www.birdchannel.com/bird-beh...d-small-bird-behavior/hormonal-cockatiel.aspx . Also how big is their cage mabey they are starting to get a bit cage territorial with each other? If they keep fighting they may have to be separated. Iam sure someone with more than one tiel may be able to give you some more info 
Let us know how it goes
Let us know how it goes 
