That is is. You just need to set the <localWorkers> field in you pom file as well (perhaps with a property from the settings file. This is mine on my current project. I have used this with 1.6.4 and trunk:
<plugin>
<groupId>com.totsp.gwt</groupId>
<artifactId>maven-googlewebtoolkit2-plugin</artifactId>
<version>2.0-SNAPSHOT</version>
<configuration>
<gwtVersion>1.6.4</gwtVersion>
<logLevel>INFO</logLevel>
<gen>target/gwtgen</gen>
<runTarget>/</runTarget>
<compileTargets>
<param>com.xxx.Main</param>
<param>com.xxx.Debug</param>
</compileTargets>
<extraJvmArgs>-Xmx512m -d32</extraJvmArgs>
<port>8080</port>
<localWorkers>2</localWorkers>
<noServer>true</noServer>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>test</goal>
</goals>
</execution>
</executions>
</plugin>