Is it allowed to ask the same question at different StackExchange sites?
It's permitted only in the sense that you won't get banned. But don't do it, it even if the question is on-topic on both or all sites.
See Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site? which is part of the FAQ, so pretty official.
People who take the time to answer your question might not be happy to feel like they wasted their time answering when you have already received an answer elsewhere.