I consider fighting behavior to be quick, jerky motions toward each other (reaching at each other to bite and scratch); a lot of chasing back and forth; sometimes spitting sounds; and lots of tail swishing and jumping/bouncing off the cage wall closest to the other chin. They used to go through periods of calmness and fall asleep near, almost next to each other (when their cages were side by side without any gaps in between, only a tight mesh screen with openings about 1/4" x 1/4"). There was a spot at the top of their cages with a wider gap (1" x 5") where they would reach up and actually bump noses. I would keep a close eye on them and noticed that sometimes they were fine. When they started acting up (mostly the older one), I would separate their cages to about 2" apart.
Just when I thought they were ready for the next level of introduction, the older one started with the mating calls all the time and getting sexually aroused every time I tried to pet him. I gave him extra play time to help him work off his frustrations, but it didn't seem to help. Both chins use the same play area, but we alternate their play times. If the younger one played first, the older one would spend a lot of time sniffing where the other chin had been instead of playing. When the older chin would play first he would spend more time playing and less time sniffing. (We thought using the same play area would get them used to each other's scent.)
Once, about 6 months ago (before the hypersexual behavior started), I tried having them in the hallway at the same time for about 5 minutes (it was enclosed at both ends). I was sitting in between them and the younger seemed oblivious that the other chin was there (he's the one with bad eyesight). As soon as the older chin saw him, he immediately charged at him. I intercepted him before he reached the younger one (I don't know what would have happened if I let him actually reach the younger one. All I could think of was the stories of chins losing toes in fights, and killing each other).
Like I said, maybe I was being too overprotective. We've read about other chin introductions (and seen videos online) and they all make it look so easy. Anyway, both of them seem happy right now and the older one seemed to calm down after we separated them. Today, I was able to play with him like I used to when he was young: I took him upstairs to wake up my husband and he (the chin) played hide and seek with us under the covers. The chin rolled around and was acting loving and cuddly. He used to get a lot of individual attention before we got the second chin.
I'd love to hear about other people's experience with chin social behavior and tendencies. They are quite interesting little creatures.