How about using only 1 wiki, but giving it two different looks (GUI's)?
1. Create two different hostnames,
wiki.example.com and
simplewiki.example.com, pointing
to the same server in DNS.
2. Add logic to your site that changes the GUI (or skin) depending on the hostname. This
could go in LocalSettings.php, an extension, or even the wiki's Mediawiki:Common.js
page.
Now you have the appearance of two different wikis with the same articles, but really
there is only one wiki. This is MUCH simpler than what you're proposing.
DanB