Within a given section, Apache httpd configurations are processed in the order in which they appear in the configuration. If you transpose the two definitions, the more explicit /nexus-pro will override the /nexus definition.
ProxyPass /nexus-pro http://server2:8081/nexus
ProxyPassReverse /nexus-pro http://server2:8081/nexus
ProxyPass /nexus http://server1:8080/nexus
ProxyPassReverse /nexus http://server1:8080/nexus
ProxyPreserveHost On
Read http://httpd.apache.org/docs/current/sections.html#merging for more information.