I read somewhere (can't find it now), that cross posting is not allowed or encouraged between SE sites. But I think this should be allowed.
Here are few examples.
If I have questions on Google Earth, I'll want to post it both on the SuperUser and GIS.SE sites. The faq of both sites considers this a valid question. SuperUser community accepts any valid software question and GIS.SE will take any valid GIS software question.
If I have a question on ArcObjects (an SDK for the ArcGIS suite), I'll want to post it on Stack Overflow and the GIS.SE.
If I have a question on Ubuntu options, I'll want to post it on Ubuntu, U&L.SE and SU.
OSX questions on the apple.stachexchange and SU and U&L.SE.
Benefits of doing this are rather obivous. More exposure, which will lead to better answers.
My suggestion as to how this option can be made available: When asking a question, there should a text box to specify which sites the questions should be displayed on (Similar to tagging questions). Only sites where users have an active account should be available as options in the text box. And of course, administrators and high rep users will have the ability to edit this.
If cross posted, a tag like the [migrated] or [closed] could appear at the end of the question. [cross-posted]
If Implemented, it would lead to the conundrum of assigned reps i.e. if a question posted on SU and GIS.SE were answered by people with accounts in both these places, to which accounts would the earned rep go to. The answer to this could be that user could answer from either of their accounts and decide where they'd want their rep to go.
P.S. - I'd like to cross post this question on 3-4 other meta.SE sites. :)
So one community gets effectively ignored because a large number of users are on the other.I wouldn't say it gets killed as people also migrate the other way. Experimentation shouldn't only be done after enough people are convinced, which isn't the case with these down votes. Perhaps you assume the best... :)