One of our clients has written up a good overview of how to quickly and easily turn your Joomla 1.5 installation into a multi-site setup. The basic steps:
- Setup Joomla on your main domain
- Park the second domain on your main domain
- Add a php header redirect to your template’s index.php file
If you want multiple templates for each site that is also easy to control. So if you need to control multiple sites from one Joomla installation, this will work perfectly for you.
View the writeup at docs.joomla.org.