When I tried to run 2 instances of XWiki on the same Tomcat instance, I faced a problem with instanciation of Solr which don't want to be instantiated a second time.
Looking at the problem, it seems that the problem comes from the method determineHomeDirectory() from EmbeddedSolrInstance.java file. When the first XWiki instance is start, this method is called a first time but System.getProperty(SOLR_HOME_SYSTEM_PROPERTY) returns nothing. The returned value is the default one returned by getDefaultHomeDirectory(). However, when the second instance of XWiki tries to start, System.getProperty(SOLR_HOME_SYSTEM_PROPERTY) returns the value of the first Solr instance already set up. This stop the second XWiki instance to run.
This is the Tomcat log.