FTBWiki.org has expanded and is now sharing content with the newly added ATLWiki.net
Basically, this only means 2 different main pages - everything else is the same, one MediaWiki instance is behind both domains. One user database, too. Editing an article at any domain will be shown as an edit on both. A custom MediaWiki plugin was written to let the wiki have 2 main pages work as intended.
ATLWiki uses a different color scheme (green). If you are a registered user, the wiki skin of your choice will be shown on both wikis when you log in. (Can be changed in Preferences tab). The wiki logo also depends on the website you are on.
Some of the pages have "set" urls. It means going to that page will redirect you to a certain site: either ftbwiki.org or atlwiki.net. This has been implemented for packs' articles.
Switching between ftbwiki.org and atlwiki.net can be done using the "Wikis" tab at the top bar.