OK the multi-JSF feature doesn't handle the new module structure of JBoss 7.2.0.Final (someone should create a new Jira :p )
I've copied all the "1.2" and "main" directory to the modules I've added in a separated modules directory (and added JBOSS_MODULEPATH="$JBOSS_HOME/modules:THE_PATH_TO_MY_OTHER_DIRECTORY" to standalone.conf).
Now it seems to bootstrap correctly.
But now I've got :
13:35:22,563 SEVERE [org.apache.myfaces.webapp.AbstractFacesInitializer] (ServerService Thread Pool -- 67) An error occured while initializing MyFaces: Class org.jboss.as.weld.webtier.jsf.WeldApplicationFactory is no javax.faces.application.ApplicationFactory: java.lang.IllegalArgumentException: Class org.jboss.as.weld.webtier.jsf.WeldApplicationFactory is no javax.faces.application.ApplicationFactory
at javax.faces.FactoryFinder.newFactoryInstance(FactoryFinder.java:354) [myfaces-api-2.1.10.jar:2.1.10] (...)
I've tried the workaround of Stan Silvert (by the way, modules/org/jboss/as/weld/main/modules.xml doesn't depend on javax.faces.api in JBoss 7.2.0.Final), but nothing changed, I still have got that stack...
Any idea ?