u/Metaxades

Hey, so me and my family have almost always had cats at home whit only the usual vet problems. Nowadays me and my dad have 2 female cats at home, which went okay for 4+ years. Recently, around 2/2,5 weeks ago, these cats had a massive fight after which they needed to be split up. We have tried reintroducing then together however everytime they see each other they immediately get aggressive and try to kill each other. Ive never seen this before, especially since they already knew each other for a long time without any problems. Any advice?