Help needed

I have two agapornis girls (probably), about an year old. They are sisters and they fight a lot. One of them provokes the fights (green) but the seccond one (blue) bites a lot or with more force. Green seems to have the force of bite under control. Blue bites green into her beak so she bleeds. Not feet, just her beak. We bought cage that separates in the middle, but both of them stand on the middle platform and bite eachother. We tried separated cages, but then they scream for eachother, in one cage they fight. Is there a way to stop this behavior or make blue bite with less force?