2 male chinchilla..1 humping

I recently got 2 male chinchillas (about 2 weeks ago), one 3 months, the other 5 months old. They came from the same breeder and had spent time in cages with both each other and other chins. The 5 month old is quite a bit bigger than the other.
They have seemed to gotten along well, aside from occasional nipping (or what I think is nipping, though the small chin doesn't complain aloud). Tonight after seeming quite wound up (he was bouncing all over in the cage), it appeared that he was humping the smaller chin. After the incident, I put the cage divider in to make it two levels to give some separation, and the big guy went wild. It seemed to make the little one sad as he cried out a few times, which made me feel bad.
After the big guy calmed down a bit, I removed the divider and put the ramp back in. Since then they have been sniffing each other and following one another around. The big guy did jump on the little one's back, but it was very briefly.

They have a relatively large cage, 5x3 feet, with multiple hideouts, lave ledges/chews, toys, and wooden ledges, and always have plenty of food and water. I was thinking this may be a matter of dominance, and if that is the case, is there anything I can do to help? I also thought maybe he was starting to go into puberty and that is why he is being more rambunctious than usual.

Also, there are many times were they seem to be getting along great, snuggling in one hut instead of two, hanging out on ledges together, eating side by side etc.

Any advice would be greatly appreciated as as I am a first time chin owner and a lot of this is very new to me.

Chins will establish dominance by mounting. It usually fairly peaceful from what iv seen from mine. Worked up, but peaceful/not harmful. If they are getting violent with each other tho then you have a problem.

Its also a very good idea to double and triple check to make sure they are both the same gender. Missexing chinchillas is quite common.

Chins also 'groom' each other. Which can look like nibbling to us. That is a good thing. Sign of care and affection. If they are biting then chances are the other chin will be barking and trying to get away. But its also possible that while grooming one chin might apply a little to much pressure which results in a bark.