@julian yeah, A owns the user account on A, but B might have a separate user account on B. the same logical person might control both user accounts. if identity was federated, the same credentials could be used to sign into both user accounts equally.

in other words, imagine identity server I, which is used to sign in on both A and B.

you make a post P1, which is published as R1a on A, and R1b on B. what participants need to know is that both R1a and R1b are authentic.