It is funny. When I first put my 2 boys in their divided tank, Smaug (old red fish in siggy), and Kamehameha (blue fish) in the same tank, Smaug had a fit. He would build giant bubble nests and pace the divider to show Kamehameha who was boss. Kamehameha would completely ignore him. He would flare very rarely, and was much more interested in exploring. This went on for about 1-2 months. Then, it seemed as though Kamehameha had, uh, matured. He was all about showing Smaug how bad he can be too. He flared almost constantly for about another month. I wondered where my sweet little boy had gone. :( Now, they both seem pretty chill with each other. Yes, they still flare a few times a day, but they also do their own thing and don't worry about what the other one is doing so much.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?