Awww....
I don't know about such things...were the adoptions at too young an age?
I don't think so. Same litter mates. One person adopted one, then backed out. The other person wanted both of them, but could not get one because of the other person.
So when the other person backed out, the person who initially wanted both, got the second one.
We have experience with litter mates. We have two, three and two. It would be difficult to separate them. I found the last two in my garedening shed when they were about two to three days old. Momma kitty did not like that.
So on her third litter, I let her be a momma. Too cute. Good momma, but the Floofy Jelly Beans chased her out of the yard. I guess that is not uncommon.
I really know nothing about cat psychology. Dogs try to please you; cats don't give a hoot. We just happen to have ten very healthy and active kitties.