Currently, an installed Geronimo Runtime is named "Apache Geronimo v2.0". If you want to install another runtime, say for Jetty if the first one is for Tomcat, it gets named "Apache Geronimo v2.0 2". You have no choice in the runtime time.
A server is named "Apache Geronimo v2.0 Server at localhost". You don't have a choice here either (except for host name), but you can change the name later, by double-clicking the server in the Servers View, and changing the name. There is similar capability for a installed runtime.
Steps to install a Jetty runtime after a Tom runtime is already installed:
1. From Servers view, right-click new->server
2. Click installed runtimes button
3. Click Add... button
4. Select server type (Apache Geronimo v2.0) and click Next
5. Choose directory (optionally select Jetty & click "Download and Install" button) and click Next.