In the best interest of some pairs, they have to be re-homed together. I was able to seperate many, but there were also many I couldn't.
I took in two liitle pugs once that were littermates. Both girls, both gorgeous and like peas in a pod. They shared (yes, shared!) bones, one at each end. They were so in tune with each other that many pics of have of them show them tilting their heads or looking the same identical way. There is no way on earth that I would have even considered re-homing them seperately. Sadly, Lulu, who was renamed Sidoney, was killed by a car in front of Nancy (originally named Elka) a year or 2 later.
I would have said "they MUST be re-homed together" without a second thought.
However, I know that many could be seperated easily.