Although they are herd animals, there is no guarantee that two random chins will get along. It really comes down to personality of the chins, just like two random people. I know some breeders disagree but I think the best way is to slowly introduce them, so they can build a friendship.
Start by quarantining the new chin in another room for 30 days, that gives the new chin time to settle in and for any illness they might have to show before exposing your current chin. Then after the 30 days you can bring them into the same room, slowly move the cages closer together until they are next to each other (a few inches apart so they can't hurt each other through the bars) but not showing any negative reaction. Then you can try taking them both out in a neutral area, either an area neither has been in before or clean the play area really well and put out new toys. Start with short 5-10 minutes max playtimes, try to end on a good note. If things go back put them back and give them time to cool off, if things go good you can slowly increase the playtime. The intro process can take anywhere from days to years, and some just never get along. If they don't get along you will need to keep them as two single chins, you can't force them to get along, chins can and do fight to the death.
