I've come up with the following solution:
* created a multi module maven project with two WARs and an additional integration test module
* the Integration test module uses ShrinkWrap to create the deployments from the existing WAR archives
* I just added the integration test module as the last module in the parent pom so the tests are executed last
* I added the maven failsafe plug-in to separate Unit and Integration tests (see http://www.oracle.com/technetwork/articles/java/integrationtesting-487452.html)
So far, this works fine for me.