The permanent solution is in WildFly. See Design of WildFly Multi-JSF feature. There is a bit of tweaking required to get this working on AS7.
Bruno, I think you used a version of jboss-jsf-installer that is meant for WildFly. The web-common module only exists in WildFly.
I've added a tag to the jboss-jsf-installer that works better with AS7. To use this version of the installer you will need to tell it the exact version of AS7 you are running. For example,
mvn -Djsf-version=2.1.8 -Dversion.jboss.as=7.2.0.Final -Pmyfaces clean assembly:single
To get this working in AS7 with CDI, you also need make some manual changes. See my earlier comment at https://community.jboss.org/wiki/DesignOfAS7Multi-JSFFeature#comment-11484. You'll need to make module.xml changes to get it working with Weld/CDI on AS7.