You can have only one IDEA project per window, but a single project can contain multiple Maven projects, see my answer in another post.
Open your first pom.xml via File > Open
to create a new IDEA project, then open the Maven Projects
view on the right, click on the green plus to add your second Maven project, and voila!
Once this is configured, you can create a new run/debug configuration which will deploy both artifacts on the same tomcat instance.