What I want to do is redirect www.domain2.com to www.domain1.com/domain2.
Then you don’t want to do it via htaccess. You need to use the WordPress MU Domain Mapping plugin.
Map any blog/site on a WordPressMU or WordPress 3.X network to an external domain.
Also see: WordPress 3.0: Multisite Domain Mapping Tutorial
The other day, Klint Finley wrote a very good walkthrough of using the new Multisite functionality of WordPress 3.0. In the comments, a lot of people wanted to know how to use your own domain names. Since I’m doing that now, here’s a quick walkthrough/how-to guide.