<p>It's in the first page of the help -- <a href="https://stats.stackexchange.com/help/on-topic">help/on-topic</a>, third-last paragraph says:</p>

<blockquote>
  <p>Please note, however, that cross-posting is not encouraged on SE sites. Choose one best location to post your question. Later, if it proves better suited on another site, it can be migrated.</p>
</blockquote>

<p>And broadly speaking, this is the position. However, it's mitigated somewhat by meta-posts here that suggest that if you expect different <em>kinds</em> of answers (e.g. seeking legal aspects of a question vs interpersonal aspects would each be on topic in their respective SE sites); in that case the kinds of answers sought would be fundamentally different and we'd see that as reasonable. </p>
