Can I post the same question in 2 communities if it is related to both?

Question

I have a question that would fit in 2 communities : Information Security and Cryptography.

Can I post it in both?

Answer 1

Generally speaking, cross-posting the same question on multiple sites is frowned upon. As discussed on an FAQ post at the main meta site here:
Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site?

The suggestion is that you ask where you feel the question fits the best. If you receive little or no response then you can request that a moderator migrates it for you.

Answer 2

Because Crypto and Infosec are so closely related, we typically split questions along the lines of:

• Crypto: mathematics, theory, challenges etc
• Infosec: implementation, usage etc

But don't worry if you post on the "wrong" site we share a lot of the same community individuals, so it is likely to still be answered, and we can migrate if needed, as @Chenmunka said.