How do you ask a question on Ask Ubuntu, Server Fault, the Unix and Linux SE, and Super User all at once? My particular question is about Ubuntu so I put it there, but it could easily fit at any of the others.
The simple answer: Don't.
The more detailed answer: Each SE has a specific set of FAQ and guidelines that specify what kind of questions may be appropriate there, and yes, there's a lot of overlap between the different SE sites. Your example is likely a usage issue of Apache on Ubuntu, so my gut feeling tells me that asking the question on Ask Ubuntu would be better than spreading it out everywhere.
This also encourages you to have a narrow scope with your question. This improves the quality and usefulness of the question, and has an effect on the answers, too.
You should choose the Stack Exchange site where you think your question has more chance to get an answer between those sites where the question is on-topic.
If the question is about different aspects, for which a different Stack Exchange is appropriate, then you should split the question in different questions you ask in different Stack Exchange sites. You should not ask the same exact question on different sites, but rather tailor the question for the site where you are asking it.