Just use one entry in setttings.xml like that
<server>
<id>nexus</id>
<username>deployment</username>
<password>deployment123</password>
</server>
and then in distributionManagement in your pom.xml's you use something like that
<distributionManagement>
<repository>
<id>nexus</id>
<name>Nexus Releases</name>
<url>http://localhost:8081/nexus/content/repositories/releases</url>
</repository>
<snapshotRepository>
<id>nexus</id>
<name>Nexus Snapshot</name>
<url>http://localhost:8081/nexus/content/repositories/snapshots</url>
</snapshotRepository>
</distributionManagement>
For fully working setup with this look at the Nexus Book Examples project that are used in the trial guide. You can add a site with the same id as well, of course. Keep in mind that there is no problem if the id;s are the same as they just detail the identifier of the server element in settings to look for and are NOT an id element for the repository. Imho it should be called serverId or something to be clearer, but thats a different story.