My first cat, who lived to be a little over 17 years old, from a young age acted very territorial/ upset whenever she saw cats outside the apartment window - her fur would bristle up, she'd start breathing heavily, would sometimes violently bat her paw at the window screen. So I always took that as an indication that getting another cat would be a very rough go. So, it was always just her and I, which was fine, as I loved her/bonded, but I also always felt bad leaving her alone when I went away on my many day or weekend trips or simply leaving for work. As, I do think she was probably lonely.
So I always had it in my head that once she passed, I was going to get a young kitten along with a 2-4 year old cat who needed a home. So, I did that, and have had the 2+ year old since mid June and the younger 4.5 month since ten days after the first. I was very careful with an initial separation and gradual shared space, and perhaps I had some luck combined with doing the slow intro, as so far things are smooth and I haven't heard a hiss since those first ten days of introduction. They play a lot, the older grooms the younger occasionally (though with time she has backed off doing that as much, as the younger isn't often receptive to that), very occasionally there's mutual grooming that goes on, and though they never curl up next to each other for long sleeps, they'll sleep peaceably near each other. It just seems they have a good respectful dynamic and I like to think they both get something out of it. Too, I think they both enjoy having a playmate when they're both keyed up and wanting to wrestle/expend some energy. I'm kind of curious to see how things evolve over time. Hopefully positively!