We had Willow for 2 years, since she was a kitten, when we brought Buffy home. Willow hated other cats and we didn't think it would be a good idea to bring Buffy home, but she was only 4 weeks old and was going to be put down the next day.
What we did was we held them both, and let them sniff each other (keeping care to hold down the paws). Buffy mostly just screamed, being a hungry baby, and Willow hissed something fierce. We soon just held Buffy all the time, while Willow was allowed to remain in the same room. Willow soon got used to Buffy's smell and screams, and resorted to turning her back to us (she'd take one good look to see if we were holding that black intruder that day, and then would sit down, her back facing us). Then one day I decided to let them both go, both on the floor with nobody holding them. Willow was put into the harness with a leash, in case she decided to try anything nasty (she once chased another cat around the house until she cornered her under a bed). Buffy was her playful self, until she saw Willow at ground level with her. Buffy was bold and walked straight up to her, and tried to sniff her, touch noses. Willow tried too, keeping her belly low to the floor. You know how cats sometimes try to sniff something, and flinch when they touch the object with their nose? This happened, when they both flinched when they touched noses, and i was tense, so I accidenly flicked the leash. That spooked Willow enough that she flew 10 feet up in the air (and she was underneath a 3 foot table). Willow fell backwards, and slipped out of the harness in the process. When Willow crashed into the table, Buffy ran away, and Willow ran in the opposite direction.
Later that night, Buffy was playing in a basket, and Willow was watching from the other side of the room. In the basket was a stray cheesie, and Buffy started eating it. Willow, being a pig herself, heard the crunching and decided to see what it was. She couldn't see into the basket, though, so she flipped onto her back and started pawing at Buffy's tail, which was hanging out of the basket. Buffy didn't notice, until she turned her head just an inch and saw that big beefy paw playing with her tail. She spun around and hissed in fear, though it sounded more like a gasp to me. Willow stayed on her back, wondering where the tail went off too, and Buffy started hissing nonstop, fascinated by the new sound she'd just made. Soon she growled and was fascinated by that too. Willow was quickly forgotten by her, as she tested out the new sounds. Soon, she was getting cranky so we moved her off to a room and praised Willow.
Later on, we let them meet up again, and it was a similar result. Buffy was unsure about Willow, and the same went with Willow being unsure about Buffy. Over this time (ranging about a week) Willow discovered that this baby wasn't going to hurt her or her "territory". So she watched Buffy alot more, and if Buffy ran out of the room, chasing something (or was just hungry), Willow followed, curious. Buffy soon became accustomed to the big grey cat following her everywhere, and they started sniffing each other more, and then they finally had a breakthrough. They started to play together, though both were wary at first, not sure how the other would treat them.
They kept getting more and more used to each other, and Willow quickly became a mother to Buffy. Buffy lost hers at 4 weeks (momma died and siblings did too) and Willow made sure Buffy was clean as soap, and would wash her if Buffy didn't go over herself thoroughly. Willow taught Buffy the kitty things, such as how to roll on the sidewalk and scratch those itchy places, and Buffy taught Willow how rolling in dirt is fun too (though Willow never ever tried rolling in dirt. She's much too clean and ladylike for that). Buffy also reminded her of how to climb trees, and Willow began to gain confidence in herself, and she mellowed out a bit. She no longer follows strangers like a hawk, and has stopped being so "evil" lately. Willow has also "risked" her life for Buffy, as she would prefer to think. She used to like Jake, the dog, until she once fell out of a window and landed beside him, which scared both of them and resulted in a chase that ended with Willow stuck in a tree.
She then always hunkered down when Jake came around, as if she could make herself invisible. But one day Jake was inside and we were trying to carry Buffy into a room. Willow saw Jake scrambling and scratching at the floor, trying to follow her as I held his collar. Willow suddenly came out of nowhere and attacked him (my legs were involved in the attack, and still have scars, lol). Jake didn't notice her, even when she gave a demon growl. She then attacked him again, seeing that his attention was focussed on Buffy. He then noticed her, when she ran to the other side of the hall. She gave another demon growl, and my sister came to see what was going on. All of a sudden, Willow hopped towards him (hopping sideways) and then ran at him full-out. She attached herself to his face like velcro, as Jake thrashed and tried to get her off. my sister was screaming and trying to separate them, as I pulled Jake backwards as he was trying to run down the hall. She soon let go and Jake's face was bleeding.
Everyone was okay, but Willow discovered that she could protect Buffy that way, that she can still hurt the dog that is 10 times her size. She has since tried attacking him at least 4 times since that first incident 5 months ago. The last time was when we brought Jake in and were trying to teach him to live with cats, and Willow was getting a bit antsy about how he was staring at Buffy. Willow was beside me, on the top of the couch. I looked at her, she was lying there just staring at him. I looked at Jake, then looked back at Willow and she was half off the couch, poised and ready to spring on Jake. We had to immediately call mom to go get Willow, and she then hissed as Mom picked her up. It's sweet that this cat that once hated Buffy now is her protector.
Sorry that was so long, lol! You can try separating them at first, let both of them get used to their new house. Soon, let them meet for a brief period, and take them away from each other if they show agression. Soon, lengthen the period of time that they can see each other, and then let them interact. They may fight, and I think you should let them work it out. If one is going at the throat, or one is screaming in pain or fear, then you should step in. But they'll need to determine who is going to be the boss-cat, and if you don't let them figure it out, they'll fight for the years to come.