I had 2 bonded boys and Baby died suddenly and unexpectedly. Mr. Whiskers and I were both very depressed and mourned his death together.
He would just stay in his cuddle cup with his little head on his paws hanging over the edge. I sat there crying and petting him. After about a month, I decided to get him a kit, hoping that would make him feel better and like the "Big brother".
After quarantine, I was able to introduce them and they became bonded. They were both neutered (Whiskers and Baby were together and had to be neutered for fighting, but they were successfully rebonded. The Snuggler was also neutered because he started fighting with Mr. Whiskers.).
Mr. Whiskers and The Snuggler are both still happily together, but I remain forever vigilant for signs of trouble.
I anthropomorphize my chinchillas, so I always talk to them. I know they understand. Baby died in my arms, so I held him in the cage and let Whiskers smell his limp body. I told him that Baby was never going to come home again. I knew that I was taking him to the vet for an autopsy, because it was such a sudden death.
I agree...see how Serge does. Be sure to tell him what happened when Teecho dies. Let him smell the body and say his goodbyes as you are telling him that Teecho won't be coming home agin and why.
My heart goes out to you. This will be very difficult for you, Teecho and Serge.