I have had to deal with bullied chins twice, both bonded pairs.
The minute blood is drawn, you can no longer trust them together.
I had no idea that Mr. Whiskers was bullying Baby, until I came home one night and found Baby cowering in the corner with most of the fur ripped from his back by his heretofore loving "brother" (they were unrelated), Mr. Whiskers, who himself got away with a pierced and bloody ear.
I immediately scooped them up and took them to my vet (Dr. Kevin Fitzgerald of Animal Planet's show, Emergency Vets) who suggested that they both be neutered to take the testosterone out of the mix and try to rebond them later.
I agreed, kept them separated until surgery day, no matter how sad they looked to be apart.
Since they both went through it at the same time, he said that they could be together while they recovered to give each other moral support. Fortunately, they both recovered nicely with only Mr. Whiskers getting a little dehydrated.
They lived happily ever after until Baby died suddenly and unexpectedly from completely unrelated causes.
Long story short, got a new young chin, My Little Snuggler, to live with Whiskers.
They were quarantined and introduced over a very long time and then they bonded.
Later, Mr. Whiskers, being older and neutered was then himself badly attacked by The Snuggler, who was reaching adolescence and getting bossy, so again I came home to a bald and battered Whiskers cowering in the corner. It was then The Snuggler's turn to be fitted with his "Little Pink Apron".
Dr. Fitzgerald again recommended neutering The Snuggler, who said of him, "you know, those testicles are nothing but trouble"!!! I asked if he was speaking from experience and he laughed and said yes.
Once again, recovery and then a long slow intro to rebond them.
I was very fortunate that both battered chins survived -- others on this forum have had long time bonded chins killed by their cagemate.
Although I was very lucky, I remain vigilant, because I know that CAN happen again, without warning. I keep extra cages for immediate separation should the need arise.
Thanks for reading this long post.
Good luck with your babies...
ETA: many people are afraid to neuter chins because they are so small, but if the surgery is done by an experienced Exotic specialist, it is very safe.
