a little story

we have a cat ( Female) for like 2~3years

but now we just got an other one Female like 6YO what we should do so they dont fight?
or kill each other lol

I had this same problem only with 2 males so I'm sot sure how helpful this will be.

Both of my male cats are about the same age, but from different litters. We had Bennie first but it was only about 5 months until we got another male cat Snickers. When we brought Snickers home for the first time they would do exactly what your saying, fight, hiss, bat at eachother. The works. I think this is due to the fact that your cat that you had for 2~3 years feels like it's home is getting invaded. Remember cats are very territorial creatures.

We decided to try and separate them and only let them hang out together in a supervised setting for a few hours a day until they got used to seeing each other.

After like 6 months of separation we kinda just made them stay together and work out their issues with each other.

Now they are best friends and they love each other. I bet your cats will warm up to each other, you just might have to give it time

Wish the best to you and your cats!

we gave a try on that but one of them scream like hell and its freaking loud

i never found a way to get my male bunnies to get along, dunno how cats are. any chance you can put them in separate cages next to each other so they're more comfortable?

its rare that 2 females cat not from same family get along well.
Get ready to see them fighting all the time until the dominant one establish the boundary

Prolly put onions and garlic around each others neck.
will avoid