If you are confident that a question is on-topic for a specific site, go ahead and ask the question on that site. Don't cross-post the same question on multiple sites.
In theory, the site with the most active users will be most likely to answer your question (and do so more quickly) than sites with less active users. However, if your question is a "fringe" question for a particular site, its users may not have enough expertise to answer your question, so in some cases it makes sense to post your question, intentionally, on a site with fewer users but more specialized users.
There are really, then, two parts to this:
- Where can I post (without having my question closed/deleted)?
- Where should I post to maximize my chances of getting a good answer, quickly?
Where Can I Post
Each site's Help Center details the topicality rules of that particular site. These are further narrowed down by a long laundry list of meta questions on each site.
As long as you ask questions on a site where you can ask them, you're not doing anything inherently wrong - you just might not get the answer you're looking for if you ask on the wrong site. Or it may take a very long time to get the right answer.
Where Should I Post
This is really a matter of experience and preference. As long as you can post a question on a given site, you have to weigh the factors against each other:
- How many active users does this community have?
- How specialized is my question? -vs- How specialized is the user community on this site?
You can find the number of active users here for each site.
When it comes down to it, though, you really have to know what your question is about.
Usually there is a ton of information a user can give when they have a technology-related question:
- The operating system and version they're using
- If it's hosted (e.g. on the cloud or a private server), what kind of service it's being hosted on, e.g. Microsoft Azure or Amazon EC2
- The program(s) they're using and versions, bitness, etc.
- The hardware they're running on (if applicable)
- The programming language they're using, if they're coding
- The problem domain (mathematics, physics, home networking, system administration, PC gaming, etc.)
But ultimately, the core of the question is going to boil down to a small number (usually 1 or 2) of extremely relevant details out of that long list. The rest is just noise. If you ask a question on a site where the topic of that site is part of the "noise", you're not going to get very good answers.
How do I peel an apple without removing too much of the meat? I have a Fisker brand paring knife, a Red Delicious apple, I live in Mexico in a small single-story apartment, I have 3 kids, and all 10 fingers. My hands fit in "Large" size gloves. I generally try to consume around 2000 calories per day.
You might laugh at some of the details provided in that question, but that's what many IT questions sound like. Based on the above, would you ask your question on any of the following Stack Exchange sites, which either exist or don't exist:
- Gloves Stack Exchange
- Parenting Stack Exchange
- Fine Motor Skills Stack Exchange
- Apples Stack Exchange
- Food Preparation Stack Exchange
- Mexico Stack Exchange
- Apartment-dwellers Stack Exchange
- Knives Stack Exchange
- Fingers Stack Exchange
Since this is a fairly easy problem to understand, most people would agree that out of the above list, you'd either want to ask on Fine Motor Skills Stack Exchange or Food Preparation Stack Exchange (or maybe Apples Stack Exchange). You certainly wouldn't want to ask on Mexico.SE, Apartment-dwellers.SE, or Parenting.SE.
...And yet, in IT, people understand the nature of the problem they're having so poorly that they're very likely to ask that question on Mexico.SE. I don't know how to solve that problem of education without delving into politics and other such controversial topics. But such is the status quo. Don't be one of those people and you'll be fine.
About Your Proposed Cross-Posting Feature
The general idea, as I understand it, is to make it so that you can take a given question and post it once while having it appear on multiple sites. Then:
- The comments made would originate from users of any of the respective sites, and all the comments would be "merged" together.
- The answers added would be merged together, regardless of which site they originated from.
- If the question was off-topic for a site, it'd be clos-- wait, what?
Do you see where this starts to break down? There are a bunch of problems with this that cross-cut the very structure of how Stack Exchange works:
- If the question is deleted, where does it get deleted? Everywhere, or just on the site where the site's users voted to close/delete it?
- If I have a user account on multiple sites and I want to vote to close or delete the question, which site does my vote get registered on?
- Is the question ID for each site different, or the same? If it's the same ID, how do you avoid conflicts across sites?
- If the question is upvoted/downvoted, which site's user gets the reputation change?
More importantly however, why bother having different SE sites if you're going to add a feature like this? Why not just have one gigantic stackoverflow for all technology-related questions?
Well, we already tried that. You don't even need to wonder what the world would be like if you merged all the SE sites into one, because Stack Overflow is so popular that its user community already does that. And there are so many millions of off-topic questions that the users who attempt to improve the site do not have the time and attention span to go through and close all the off-topic and duplicate questions. You can visit Stack Overflow today and see what the "IT soup" site looks like.
In fact, you can probably get away with asking just about any technology-related question on Stack Overflow and, more often than not, it won't ever be closed. That's because SO receives so many questions, and so few users are interested in cleaning up the site, that they can't ever hope to turn back the tide of questions.
So instead of asking for a feature that would make the rest of Stack Exchange just as muddled and crowded with off-topic questions as Stack Overflow, why not just analyze your question a little bit more before you ask it so you know which site to ask it on?