Yes, this is a copyright violation. The holder of the copyright is the author of the content, not Stack Exchange. Stack Exchange is only redistributing content which they have a right to redistribute but do not own.
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license.
All user content is licensed to SE by its author with a license (CC BY-SA) that requires attribution, and is licensed by SE to site viewers under that same license.
Think of it this way: if I copy your answer without attribution, is it ok as long as I'm only using your work in another Stack Exchange answer? No? Well then. Copyright law does protect you here.
Now in practice, this isn't a big deal. There is no visible author attribution for a tag wiki (unlike for questions and answers where the main author is prominently visible when viewing the post normally). So you don't have to clutter the tag wiki with an author attribution just to credit Stack Exchange contributors. It would be acceptable to have the attribution in the edit comment, which is visible whenever one views the list of authors of the tag wiki (viz, the tag wiki's revision history).
In principle, you should credit all authors and link to their Arqade profile. In practice, when do that, I tend to only leave a link to the original tag wiki, which is a lot simpler. This is technically wrong because the other tag could be deleted, or the tag wiki could be included in a dump that doesn't include the original site.
This is a problem for the initial content because you can't have an edit comment. Make some cosmetic edit and write a comment on the second revision, I guess.
It's actually rare that the same tag wiki is appropriate on two sites. Different communities tend to require different information. Often, the other site's content is a good source of information but needs to be both filtered for community relevance and expanded for community relevance.