Migration and cross-posting seem to be popular topics on meta. There is also often a great deal of discussion in the comments of a migrated question about whether it was migrated correctly or most optimally. It's clearly a problem that has not yet been solved.
I have recently come across a few questions that don't fit neatly into any one particular SE site. Some questions require the expertise from two different sites to answer completely and correctly. By only receiving the input from one site, the question will result in a suboptimal answer.
This one for example was already migrated from StackOverflow to ServerFault but could easily have been migrated to Security.SE instead. Broadly speaking, the security guys will have more experience dealing with security incidents and are more likely to know about possible Apache exploits but the ServerFault guys will have more experience with the practical aspects of taking servers offline, keeping the services running and restoring from backups. (Ignoring for now that many users have accounts on both of these two sites.)
This one could easily fit on SuperUser or ServerFault. It's a question about a Mac on a Windows network. This one on Fitness.SE could easily be on Bicycles.SE and is almost identical to this one that already is on Bicycles.SE. In this question, a moderator suggested cross-posting as the best option.
A simple way to find these sorts of questions is to go to any SE site and search for the topic of a different SE site. (Security tends to be a common theme in questions in all SE sites.)
It becomes more obvious when a sub-group splits out of an existing group such as Security.SE and Crypto.SE or Unix.SE and Ubuntu.SE. There are plenty of questions both on here and on the various meta sites asking if old questions should be migrated now that a more specific site exists.
Cross posting has the problem that you end up with two (or more) different sets of answers on different sites. The original asker has to maintain all of them and the various answerers don't get to read other answers. It is discouraged on SE.
Migrating a question has the problem that wherever the question ends up, the answers won't represent all areas of expertise. Although there is a trail left from the old site, if a question is only migrated once or not at all (e.g. The first question I mentioned never made it to Security.SE) then the question will never see that expertise.
An option to post a question so that it was visible on two (or more) sites at once but so that answers and comments from any site would be visible on all sites. A solution like that would solve both the problems with cross-posting and migrating. This could possibly be called "breadcrumbs" because when you are asking a question on one site, you are also leaving trails on other relevant sites leading to the question. Moderators should be able to add breadcrumbs from any relevant site to any question.
An alternative without adding any new code to StackExchange is to encourage moderators to migrate a question whenever they think it might benefit from the expertise of another site, so leaving a large trail leading to a single question. This seems inefficient but better than missing out on the expertise required to answer the question.