Very occasionally you may want to ask substantially identical questions on two sites, to reach different communities. (For example, if you're looking for a computer application to perform a certain task, and you don't mind whether it's a local application or a web application, you might ask on both Super User and Web Applications).
This is the exception rather than the rule. The question you ask has to be on-topic on both sites.
Again, if it's your question and you want it moved on second thoughts, flag a moderator and request a migration.
If you really think your question belongs on both sites, it probably doesn't.
If you really really think your question belongs on both sites, link the questions to each other. (If it's not your own question on one site, you might just leave a comment.) This way, people won't waste their time duplicating an answer already written on the other site, and people who find the question later can read both sets of answers.