In my experience cats always do better in multiples, with two adults though you need a really nice gradual introduction, I've got it down to an art so I've never failed. Start with the cats completely seperate for a week or two, you need the new cat to get really comfortable with you and the house, this can take a while. Once they're relaxed and comfortable you can start taking things that smell like the other cat into the other location, a blanket or something works well. If the cats his at the new object initially then it's ok but if it really upsets them take a step back and go back to just getting comfy and then introduce things that are only lightly scented like food bowls.
Once you are swapping stuff consistently and both cats are relaxed then you can actually swap the cats. Do not let them see each other at this point, put one cat away somewhere, let the other into the first cats area, then switch the first cat to the second cats area. Again, if they're really upset then swap back and slow down, but a little unsure is ok. Swap them back and forth regularly for a while until everyone is calm and comfy.
Then you can start introductions through a barrier. The ideal situation is a glass door, but a door held slightly ajar can work too. Do not encourage the cats to come up to the barrier or anything, just let them come in their own time, and as with every stage, if they're stressed go back a step.
Only once everyone is happy with being swapped about, seeing each other through the barrier, using the same stuff, then you can try putting them together, and again, step back if needed.
Sometimes I can do all this in a few weeks, sometimes it takes months, but it works every time.