This question already has an answer here:
I think that now that we have multiple communities, there are questions that will be of interest to multiple communities.
For example consider the question "Homemade sports drinks" asked on bicycles stack exchange . I think that this question would be better answered on the food and cooking stack exchange (that is where the experts are). But since questions is of interest to both communities I think that it is useful to cross post it, where answers are also shared between the different sites.
I think that the best way to handle cases like this is by allowing cross posting. I think users with 500+ reputation should be allowed to cross post questions. This would allow users from both sites to answer them. I think that this could also be a good way of marketing different sites to different users.
Similar questions have been asked about the SO, SU and SF triad but I think that this has become a larger issue (and will become a bigger issue as more area51 sites evolve).