Feature-Request / Idea:
Sometimes a question seems to fall in between two sites. An example might be php email questions, could be the code or could be the php.ini which often ends up being a sysadmin. So my idea is that there could be a check box to show it on another site as well. It would be grayed out on the other site and only visible to people with high rep, and they could approve it, and then everyone would be able to see it on the other site as well. Maybe they could add answers, or maybe it would just be visible and people could go to the other site to add the answers.
So for example, if the question was posted on stackoverflow, there could be a 'show on serverfault' check box. Which server-fault moderators / high rep users could vote to be shown there as well. These could also maybe be community-wiki so simplify the rep question.
One downside I can think of is that maybe this might create more confusion about what to put what where worse, and maybe that would outweigh any benefits.
Maybe it has been thought of, but just thought I would share the idea...