Way to make sun conure stop attacking our new sun?

So I have a 16 year old male sun conjure named Puffy who I've owned his whole life. Recently, I offered to temporarily "babysit"a two year old female sun conure who belongs to the daughter of my dentist's receptionist, for maybe as much as several months, because they just had a newborn baby. For the first two months they brought food every month but for the last 9 months I haven't heard a word from them. They don't return calls or texts, and I've been told to accept the bird is likely mine now, which is fine because she's a very sweet bird, nice to everyone, very much a "people" bird. However, MY sun conure is not nice, and for some reason HATES the new sun conure. He really wants to kill her. Sometimes they have these mid-flight fights, with one chasing the other that I have to break up. Basically they can't be in the same room together unless one is locked in their cage. This even after almost a year of having their cages right next to each other, I would have thought they would have become used to each other by now. My question is, is it possible for them to ever get along? Puffy gets a long just fine with our GCC.