if you have access to a cage that is divided that would be a good way to get them used to each other. i have a cage that is 2 stories with a grate that i can put in to divide the top and bottom story so they can look at each other through it and sniff but can't hurt each other. when i get new ones that i want to put together i'll put one on top and one on the bottom and switch them every couple days for 2-3 weeks as well as supervised playtimes. then when you do decide to put them together freely in the cage, do it supervised for at least a week to make sure there are no fights, and separate them at night and when you are not able to watch them.
if you have something like a very small house or a coffee can or something that only one of them can fit into and they are not able to stand up in, put that in there as a safe area that one can run and hide in if necessary. chins mostly fight standing up or on top of each other so if one is hiding in a space where the other chin can't get on top of it or stand up that should help to protect it. if you have a cage with adjustable shelves, putting one really high (too high to stand) can also be a preventative measure.
you also might want to hold off on playtime for a while. a 3 month old chinchilla getting pregnant is about the same as an 11 year old girl getting pregnant, it can happen but it's not healthy for anyone. if you start with them in connected cages now and switch them back and forth so they get used to living in cages with the other's scent on it, by the time she's ready to start breeding it should be easier to put them together.