When Asking a Question, the suggested dupes shown in Related Questions should come from across all related SOI sites.
This would help users figure out if they are asking a question on the wrong site. It would also help prevent the duplicate-asking of questions which might be found on both sites.
This is especially important for SuperUser and ServerFault, where the distinction is not clear to new users and where many questions overlap and preemptive elimination of redundancy would be quite helpful.
Eventually, tags could even be used to discover what is related. Tagging a question Linux or Ubuntu or Debian could cause searching of, or increase the chance of results appearing from, the Unix/Linux and Ubuntu SE sites, for instance.