This seems like a really interesting idea, and in some respects could be useful, but I see a few problems which (currently, for me) stop it being worthwhile, or even useful.
I'm not sure what benefits there are, as you're only argument seems to be:
A question may have a topic that is covered by multiple sites
This is very rare, and even then a question which could arguably be on 2 sites nearly always has a specific scope which would be more suitable for one site or another. e.g. One site has more traffic/activity for your tag(s) than the other.
or contains parts that would belong to different sites.
This is a null argument, as then it's simply 2 questions on 2 different sites.
These arguments do not give much hope for your idea as sites have tight niches and questions are nearly always suitable for 1 site only.
People will (or at least could) use this incorrectly:
- By mistake, as it's not clear which sites have an acceptable
- On purpose, as they don't care about the sites and just want their
- Because we said they could and it is a juicy feature to gain a
It's hard to have stats or figures to identify how much of a problem the above will be. However based on experience I strongly suspect that when you (essentially) state something like:
Use this tickbox to have your question on X and Y site also
Then people will use it - whether it's appropriate or not.
And that would mean "clean up" and moderation (somehow), and given the fact the usage of this would be rare, the problems caused even by needing to clean up grossly outweigh the occasional and weak benefit gained.
Not very useful:
All sites are pretty tight in what is on-topic. There is some overlap in some sites, but not enough to be an actual problem and not enough to warrant your request.
A question needs to be functional on that site:
If there is no actual question on the site with the shared link, then what is the point if people on siteX (shared link) cannot answer on that site and have to go to another site altogether to answer/comment/vote/flag/etc?
While we don't have to figure out the technical aspects to feature requests, "off-topic shared links" is something which would be a major issue worth mentioning:
It should not create a clone question but instead link to the original
So on the site where the shared link is, do people have to go to another site to flag it being off topic on the site they were on?
This could be resolved, but then you introduce more functionality and technical aspects, and again for so little gain.
It's impossible to identify which other sites a question could be suitable for, so we'd have to allow the ability to push to all other sites.
This is bad because people don't know what other sites they should push to, especially as it will be rare when they can do this correctly. This is why the sites limit migration to very few scenarios.
Do we allow them to post on all sites at once? Or limit them to "X other sites"? The latter is not ideal because we do not know what their question is so we could be allowing them to post to too many other sites, or not enough.
Imagine how easy we'd make it for nuisance and spammy users to post their crap to various sites in one go. Sure this can happen with other means, but when the proposed feature will be very rarely useful, potential problems can easily outweigh the benefits.