The first step would be to configure the existing instance to listen to only one IP interface.
To do that you need to edit the job_controller.cnf and dispatcher.cnf files and uncomment the interface_address options and add the IP you want this instance to listen on.
Then recompile the configuration and restart the dispatcher and job_controller. Or do a ./imsimta refresh command.
Now, run the setup program again and install into a new server root. When the install is done follow the procedure above to make the new instance listen on it's own interface. Once you've done that go ahead and start it up.
This is the procedure in a nutshell. Reply back if you need a step explained in more detail.
-Chris