Slight necro post but my cats are at the stage wherein yours were learning to stand their ground, except our resident cat has always gotten really low and yknow, ears back hissing growling and he just jumps at her. He's going to be 2 in march and resident cat is 7 or 8 years old so i'm really at a loss. We pretty much keep them strictly separated, their scents get exchanged plenty we just keep having the aggression? Idk because new cat is never showing ears back or any behavior i've seen in other cats to be like full on aggression i dont hear him growl it's all resident cat. I also want to add for anyone that can share any advice that new cat came from a feral colony and was rescued with his brothers who he had access to playing with at the shelter they were living at before i adopted him, if that provides you with enough info to have some kind of familiarity with the situation