Maude actually accepted Attitude(female) a lot better than Nuts. She really seemed to hate Nuts to the point of not wanting him near her and she would hiss at him and wanted nothing to do with him. She was more accepting of Attitude, she would let her sleep against her. They were never friends but more or less accepted each other. Attitude and Nuts are litter mates and I have always called them 'partners in crime', if you needed to find one, just find the other and you had your kitten.
You can see from my sig pic that Maude really didn't like either one(the simple look of disgust on her face) but she accepted them before she went over the bridge. I will say Nuts seems to be more upset about Maude no longer being here than Attitude, which is funny considering Maude REALLY REALLY did not like him. Even though both have decided to take her place in their own way, Nuts has become a total lap cat(Maude got like that after Molly, Paul's dog went over the bridge( and Attitude has decided to become 'fat cat'(Maude's nickname was fat cat, and she was and Attitude is big and fat, Attitude is on a diet so I'm not sitting around letting her expand her waistline.)
Depending on the cat it can take time. The first time I brought Attitude in the house(before I knew Attitude had leukemia and put her back outside) Maude was so mad she attacked me. After Paul and I decided to get Maude vaccinated against FeLV and bring Attitude and Nuts in. We got Maude vaccinated and Paul just opened the door and let them out(I was going to give them more time to get to know each other through the bathroom door.) Maude walked by the door(don't know if she looked inside or not) and hissed. I go into the bathroom(for it's intended purpose) and Nuts is arched and puffed, now keep in mind Maude walked by and hissed, she was already in the living room. After that it was some hissing, a few swipes, and Maude did at one point take their heads(one at a time obviously) into her mouth and yes their entire heads fit in her mouth to tell them to knock something off and she was really mad. Can't remember what Nuts did but Attitude hopped on Paul's lap and Maude was already there so Attitude(having an Attitude) kept sticking her tail in Maude's mouth so she couldn't hiss at her. I will say Nuts only puffed the one time and he stopped after that(he used to puff and hiss at the Siamese that all the outside cats hated, it was funny seeing a 4 week old kitten arched and puffed up and hissing at an adult cat, they all hated her and the adults would chase her off if they saw her.)
I will say on Attitude and Nuts' first night they refused to enter the living room, I'm assuming because Maude was there and they were afraid of her or they thought it was hers and they weren't allowed in, but I think they were just afraid of Maude(she was 14 lbs and they were maybe 4 lbs at most.) Once I carried them both in they understood they were allowed in there and started coming in there and ignoring Maude hissing at them. I saw them in the hall almost in there but they didn't want to come in, like I said I had to carry them in and then they were fine. There were a few swipes but never any claws or fights.
I think it was just that Nuts was more annoying, when Mama, their grandma, was in here after her prolapse she bopped Nuts on the head more than once(and basically ignored Attitude) just like Maude did so I think Nuts was just annoying to them. Maude and Mama just basically accepted each other(I let Mama out of the bathroom before it started permanently stinking like a litter box) after Mama scared Maude(keep in mind Mama is 5 lbs soaking wet and Maude was a good 14 lbs, so if they got into it there is no doubt who would win) by hissing and beating her paw in the hallway when they saw each other the first time. Mama was older, 3 or 4 and Maude was 10 so they didn't bother each other and Mama refused to jump up on the couch or the furniture, I'm assuming her last owner(who dumped her unspayed once they were done with her) didn't allow her on the furniture because she would not jump up. I know she was owned at some point since she knows what a litter box is, how to use one, and that if you are in the house that is the only place you go.
It might take time or they could be immediate best friends or they might simply tolerate each other. I didn't ever do it the 'right' way when introducing them(I did however put Attitude's scent under Maude's food bowl) but it worked in my situation, if there had been any fights they would have been separated a little while and then try again later. You will need to be able to be home for the initial introduction period. You can do it the 'right' way or wing it like I did. You never know how they will react to each other and how much time it will take before you can safely leave them alone together. I will say Paul and I left the trailer the first day for several hours with all of them roaming around, but we knew they weren't going to fight, we had seen the initial interactions and knew they'd most likely just ignore each other. They had staked their claims on their territories at that point, Maude had the living room and the kitchen(which has a litter box) and Attitude and Nuts had the bathroom(which also has a litter box) and bedroom. Also I knew Attitude and Nuts had multiple places they could escape to that Maude couldn't fit her big toe in so I knew if push came to shove(and it never did) they had places they could get to(in almost every room) that were totally inaccessible to Maude in case they needed them.
Like I said you'll have to decide, in my case 2 females meshed better than male and female.
Taryn