See tomcat7:run mojo parameters: you can configure plugin executions to use other ports, with something similar to this config:
<execution>
<id>tomcat-run</id>
<goals>
<goal>run</goal>
</goals>
<phase>pre-integration-test</phase>
<configuration>
<port>9090</port>
</configuration>
</execution>
And of course use another port for the second project.