We introduced Moon and Bean when Moon was 8 and Bean was 5 (I think). Moon had been an only child his whole life. Bean only knew other cats from his outdoor escapades. We kept them separated for 2 weeks. Moon still had the upstairs of the house and Bean got the finished basement (which he still owns, by the way... ). After 2 weeks we opened the door and Bean crept upstairs slower than any living thing I have ever seen. I think he moved a millimeter an hour...haha. When he got to the top step, he had his whole body on but not crossing the landing.

They have lived together for 2 years now and they certainly worked it out. Yours will too