There have been several really good responces. You are very lucky your two chins like each other.
Most chins will get along with each other, but, there are a few that won't get along. Each chinchilla has it's own special personality. With chinchillas, same as with humans, personalities can clash.
I would take them out together for play time, lay in the floor (very still) and let the chins come to investigate you (they are curious creatures). After they have played for a while, put one chin up and keep the other one out for some one-on-one play. Then bring the first one back out for a little while and play together again. Then, put up the one that has been out the most and play with the other one. This way, you are showing both that they are both wanted and loved and that you will allow them to play together, but that you also want to have a 'special time' with each of them individually also.
If you will make sure you stop and talk to them, regardless how briefly it may be and/or call them by name and say "Hi" or "Bye" each time you enter or leave the room ... you'll be surprised how much they pay attention to something that simple.
Keep communications open! I have over 100 chins, but many will stick their little noses through the wires to get me to rub their noses and talk to them for a few seconds. I have some that dance for me and others that do flips for me ... all are vying for a bit of attention.
SF, true, he could have gotten out by accident, but with no one answering the adds or posts, it is more likely that it was just let out and left ... such is done with many cats and dogs.
Many people have no idea that chinchillas can not live in the out-of-doors due to the weather we have here and in many places.
Jo Ann