You can't combine them.  It's a [Name-based Virtual Hosts][1] configuration.  `VirtualHost` configs are completely segregated.  What many people do is they'll set a different `VirtualHost` in different configuration files but I like to combine service related `VirtualHost` configs into the same configuration file.  You can think of them as being two separate files.  I'm not aware of any method to alias the host name.  Perhaps with an environment variable?  But that would over complicate for just two lines.

[1]: http://httpd.apache.org/docs/2.2/vhosts/index.html