I have tried this in a couple of scenarios. I.e., installing demo first then attempting to install second empty system. and installing an empty database then going back and installing a second database. I deployed the original war file under two different names: cmdb1 and cmdb2 and then attempted to install them. It always fails at the database section.
In the same Tomcat instance you can install as many CMDBuild instances you want.
Obviously you have to configure them properly, and Tomcat must have enough available memory.
CMDBuild Team