My family and I have had a 3 year old cat for a couple of months now, but a little over a week ago we adopted a 5 year old cat (both are females and spayed). We placed the new cat and her supplies in my room and introduced them slowly. We now let them roam the house together as long as someone is there to watch them (the new cat sleeps in my room at night). When they are together in the same area, our resident cat occasionally growls/hisses but that's it. Other than that, they can stay in the same area without much happening.
However, there's been instances where we saw our new cat ran up close to our resident cat and growl/howl at each other pretty loudly. Although every time that this happened, they haven't broke out into an actual fight; no biting, no hitting. Just loud cat noises.
It does seem like their relationship is slowly getting better but it's also a bit hard to tell. For example, earlier today I received a call back home and was told that my dad was playing with our new cat while our resident cat was resting at a nearby window (the window she was sitting at was pretty close to where the new cat was, so I'm pretty sure she saw my dad playing with our new cat. My cats ended up switching off aha). I was also told they were roaming the house throughout the day without any issues, but then later that night, that's when our new cat ran up to our resident cat.
Is this type of behavior normal between two new cats? My parents and I are worried that things might not get better if they keep doing this. Any advice would greatly be appreciated!
However, there's been instances where we saw our new cat ran up close to our resident cat and growl/howl at each other pretty loudly. Although every time that this happened, they haven't broke out into an actual fight; no biting, no hitting. Just loud cat noises.
It does seem like their relationship is slowly getting better but it's also a bit hard to tell. For example, earlier today I received a call back home and was told that my dad was playing with our new cat while our resident cat was resting at a nearby window (the window she was sitting at was pretty close to where the new cat was, so I'm pretty sure she saw my dad playing with our new cat. My cats ended up switching off aha). I was also told they were roaming the house throughout the day without any issues, but then later that night, that's when our new cat ran up to our resident cat.
Is this type of behavior normal between two new cats? My parents and I are worried that things might not get better if they keep doing this. Any advice would greatly be appreciated!
‘I dislike you but I will tolerate you if you stay over there’ Stage. The message that treats, play and grooming while they are in the same room as each other is ‘look good things happen around this other cat ...., maybe this other cat might be good?’ 
Except the brush thing, that is a co-mingling of scents so both cats and you have the same communal scent (cats have communal scent and their friends/family all smell alike.) You could also try a sock (washed in unscented stuff) and put your hand in it.... then pet each cat under the chin/on the cheeks to get them to smell like each other. If you think scent is unimportant to a cat, get a perm and watch how they don’t recognize you or want to be near you ..... until your hair is less smelly. 
I definitely will try these techniques out!
