You did not say which version of the portal you are using. The instructions below are for 2.4GA

First, you will need multiple instances. To create multiple instances, in the Administration Portlet, click on the Portlet link at the top of the window. Scroll down the list until you get to the CMSPortlet, clcik on it, and the at the right fill in a new instance name and click create.

Once you have the instances, you can specify a new start page as follows. In the Administration Portlet, click on the instances link at the top of the window. Scroll down until you get to your instances (they are at the end of the list). Click on the instance and then to the right you will see the indexpage preference. Click on the icon showing a pencil over a piece of paper and you can then edit the preference.

WARNING: This will not work on 2.4GA due to a bug that has not yet been resolved. See http://www.jboss.com/index.html?module=bb&op=viewtopic&t=89308. This did work, however, in 2.4CR3.