When I worked full-time, I usually had two cats so they wouldn't be alone so long when I was at work. I've had cats that were almost the same age (adopted within a week of each other), and also had cats where I had one for a few years and then introduced a kitten.
Same age is pretty easy. And if you get them at the same time, neither feels like their space has been invaded.
The hardest introduction of a kitten was (just my limited experience, nothing googled or researched), was an adult female and a male kitten. She never liked him or played with him ever. She wouldn't even eat in the same room as he did and she wouldn't let him on my bed. Male adult, female kitten, much the same, but he ate with her - he just seemed indifferent. Male adult, male kitten - took about 10 minutes of sniffing and a little hissing, then they became fast friends.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine