Compared to the others in this post, my experience with fighting cats was opposite. My family's first cat was Loki. He was an only kitten from a feral mommy kitty that roamed our farm, and after we tamed him he became an only kitty indoors for a year or two. Then my mom decided to adopt Milo.
Loki was usually a timid kitty, but he could get ornery for a golden (his mother WAS a tortie). We tried for a month to do the whole introduction thing, but he was not having it. Milo was the most relaxed cat I've ever seen- a lover, not a fighter- if you will. Every introduction ended in Loki hissing and running away. Finally, as a last resort we forced them to stay in a smaller room without hiding spaces for about two weeks. They had all of their meals and spent their entire day together, and we didn't let them out of the room until Loki finally became civil with Milo. Even then it seemed like he only tolerated the other cat in his space. However, I quickly found that he cared for Milo when I was spraying him with water for jumping on the counter and trying to get at food that wasn't his to take. He yowled and suddenly Loki came up behind me and bit me on the calf! It was totally not something he would do. When I turned to look at him, he gave me those big "uh oh, I'm in trouble" eyes and ran away. But in fact he wasn't in trouble. I was laughing so hard I had to go tell my mom about it.
I say all that to say this: Your situation is obviously a little different than mine (different cat personalities, etc), but I would say that if a reintroduction doesn't work, consider something a little non-traditional. It worked for us.