Sometimes a question can be on-topic for one site, but more likely to get answers from a different community, for example: http://electronics.stackexchange.com/questions/55492/how-to-get-mac-os-x-10-8-mountain-lion-to-see-usb-modem-device-arduino
In this case the question is on-topic for Ask Different and perhaps off-topic for Electrical Engineering. However it's likely to have more understanding and knowledge and get better answers in Electrical Engineering since it's about the Arduino. Many variants of this example case also exist where the question is on-topic on both sites or somehow sits in the middle, etc.
Something very similar to my suggestion below has been asked before: Show on 'other site' switch however I have incorporated the feedback to that suggestion into my suggestion so that it avoids the problems detailed there and hopefully makes it work better for the asker, answerers, and the communities and moderators of both sites.
When positing such a question you would post it on the site where it's more on-topic (Primary site) and select an option to create a proxy on another site (Secondary site), you would choose the other site and at the same time pick the tags from the other site.
- Posting this question would NOT create the question on the secondary site as well, it would create a proxy-object for the question and tag it with the correct tags. The primary site would post the question immediately as normal, but on the secondary site the proxy-question would need to be be moderated and approved before the proxy object becomes widely visible.
- When browsing the secondary site, users would only see proxy-questions for questions held on sites on which they already have an account (There could be a user setting to show proxies from all sites if desired).
- When clicking on a proxy-question you would be taken to the primary site in a new tab (the question is only in one place). Answers would only be posted on the primary site.
- Proxy questions could be visually distinguished from normal questions by various means: using different colours/style, an icon or obvious 'proxy' tag, putting them in a separate section, etc.
This idea of using proxy-questions may also help to solve a few other cross-site issues. For example when closing off-topic questions, moderators may also be able to convert the question into a proxy-object of a duplicate question on another site (I've seen suggestions around this too).