Hi Ross, welcome to the forum! We are glad to have you here.
I hope you have a great vet that is very knowledgeable about chinchillas because they can have some bad problems after neutering. I would not suggest having the other chinchilla done.
Females are actually the more dominate and males are easier to pair together than females. Chinchillas are just used to pattern and stability and don't like change. Any chinchilla at that age will usually fight having a cage mate. Once they have lived alone for so long they like being alone. I often have people contact me wanting a friend for their adult chinchilla and I tell them the same thing I am going to tell you. If YOU want another chinchilla for YOU it is a good idea. If you want another chinchilla for your CHINCHILLA then don't do it. The chinchilla will be stressed with the change and it doesn't always work out.
If your chinchillas are not getting long don't stress them by adding more chinchillas. Just keep them in side by side cages and enjoy them. They don't need a cage mate to be happy.
Lets look at how to better get your two boys to like each other. What exactly are they doing that you call the "alpha male"? It could be that you are interpreting normal behavior as a problem. Chinchillas will bump noses and bark at each other a lot. They will jump on each other's back a lot. (both males and females will do this). No matter what sex chinchillas you have (same sex or opposite sex) they will have to agree on who is the boss of the cage. In some situations one chinchilla is happy to step down and bonding happens in minutes or days. Usually you have two chinchillas who both want to be boss and you will see them each taking turns being alpha. This is a personality thing not a hormonal thing. It cane take weeks or months for them to live together. You just have to let them work it out for however longs it take until they come to an agreement.
You are doing great letting them out in a neutral place. If they are are rolling around fighting and pulling hair you need to step in. If they are nose to nose barking and mounting each other you need to step back and let them do their thing.
Tell us a little more about what your boys are doing and we can suggest some ways to help.
