Chinchillas tend to do very well in pairs and paired chins have several different ways in which they communicate and show affection. One way in which your chinchillas may greet each other and show affection is by nuzzling the tops of each others noses. It may look like biting, but this is a grooming affectionate gesture (unless for some reason the other one is squealing do not be alarmed.) They also like to be nuzzled under their chin (try scratching under your chinchillas neck sometime, most chinchillas LOVE it)
They will also make cooing noises at each other. This often happens when you separate two paired chinchillas and then put them back into a cage together. They "miss" one another and this cooing sound is one of relief and happiness that they are reunited. Your chinchillas will also often sleep together, one may even sleep partially on top of another chinchilla.
They will likely also develop a pecking order for eating and dusting. You may notice that one of your chinchillas always eats first or dusts first while the other waits it's turn. The one eating or dusting first has established herself as the Alfa. In one of my cages the alfa is the eldest female. She dusts, drinks water, and eats before the others. She shares her bath once she has had a few rolls first, but she is always first. If another of the chinchillas tries to jump into the dust bath before she has had enough time, she will stand up on her hind legs to assert her authority. This is usually enough to get the intruding chinchilla to back off, but if not the alfa female will chatter her teeth a little, and as a final step chase the intruding chinchilla off or act like she is mounting (humping) the other chinchilla. This interaction does not last long and the intruding chinchilla is quickly forgiven if they are cage mates.
Treats seem to be a different store. My chinchillas will go for a treat no matter who is the alfa of the cage. I give them all treats at the same time to prevent them from stealing one another's treats or fighting, but they typically will run with the treat to save it rather than give it up to another chinchilla. I have however seen adult chinchillas give up part of their treat to a squealing baby chinchilla without putting up too much of a fuss.
In my experience, kits tend to be treated well by the entire herd. I have my chinchillas paired and separated into different cages (as I have found that two males and femlaes in a cage is dangerous and inadvisable), and if a new born kit escapes it's cage and wanders into another chinchillas cage, it is not harmed or chased by the other chinchillas. I do separate my kits and the mother into a baby safe cage while she is nursing to prevent escapes, but babies can be quite the little escape artists and when I have found them in other cages the chinchillas of that cage were friendly to the kit and often would even groom her.
Females will also take care of kits even if they are not their own. In one case I had 2 females that had given birth a week or so apart. I placed both mothers with their kits in separate baby safe cages, but two of the kits escaped. I found them in the cage with the other kits mother nursing right beside him. I separated them back with their own mother, but they were content to be with either and neither mother seemed to mind nursing the others kits. Babies will also have a larger range of sounds they use to communicate with each other, their mothers and other chinchillas.
I highly recommend keeping chinchillas cages in a chinchilla safe room, not the middle of the house, especially if you have other pets. Chinchillas are highly intelligent and they tend to find tricky ways to escape from their cages. My ebony female chinchilla will actually lift the handle of her cage and kick the door open. I tried using bread ties to hold it in place, but she found her way through those as well. In the end I had to put a metal key ring on the doors to prevent her from escaping. Fortunately since she is in a small, chinchilla-proof room there is not much she can damage or get into. I went through several books before realizing that no matter how hard you try, your chinchilla will likely find some way to get out of her cage and cause a ruckus.
Watching and studying chinchilla behavior is probably one of the most rewarding aspects of owning a chinchilla. They are not affectionat in the same ways as a dog or cat, which most of us are familiar with, but with enough patience and willingness to to interact with your chinchilla at the chinchilla's pace, you can experience the joys of chinchilla affection yourself..