Can two adult cats get along?

With love and care, they will get along eventually. Just let them get used to each other. If things get bad and they actually start fighting (not playing) which you can tell that they are fighting if they are hissing or literally tearing each other apart, you must get rid of one of them. I have adopted a street cat and I have already had a house cat. They fought at first for a couple weeks, but then they got along very well, my street cat still acts like a street cat outdoors, but gets relatively close to my other cat, they play all of the time now and watch birds out of the same window, they are best friends now.