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I sometimes have questions that are suitable for 2 or more Stack Exchange sites. For example, a question on configuring a development database for a website.
Wouldn't it be a good feature if you can post the question on one site, then just register it as a 'cross-site post' so that the question and all its answers can be visible on all the sites that it is marked for.
This will cut down on cross-site posting, and allow the same question to be answered by experts with different expertise.
This will promote cross site collaboration, as different users from different sites will be able to see how other parties are answering or commenting on the same question.