The top answer works if you are not using any configuration (or any resource, for that matter) that gets bundled into your jar file (e.g., configuration for Spring Framework auto-bindings).
Fortunately, this solution also works with maven-shade-plugin and you don't have that aforementioned issue with onejar-maven-plugin.
Also, maven-shade-plugin is actively being maintained as opposed to onejar-maven-plugin which is in the purgatory that is googlecode.
org.apache.maven.plugins
maven-shade-plugin
2.4.1
build-first
package
shade
classpath.to.first.Main
first-runnable
build-second
package
shade
classpath.to.second.Main
second-runnable