This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
There's nothing stopping you from running multiple Tomcat instances, provided they have separate installation directories, and use different ports. What would be the advantage of this setup over having a single instance serving multiple web applications? Or to use multiple hosts within a Tomcat instance?
Manon, you should be able to configure multiple HTTP Connectors in your Tomcat server.xml file that listen on different ports. Refer to the Tomcat documentation for details on how to setup your server configuration to do this.