You can do this:
parent pom.xml:
<project>
...
<version>${my-project-version}</version>
...
<properties>
<my-project-version>1.1.1</my-project-version>
</properties>
</project>
child pom.xml:
<project>
...
<parent>
<relativePath>../parent/pom.xml</relativePath>
<version>${my-project-version}</version>
</parent>
...
</project>
At least, this works for me.