First you need a wildcard DNS entry and a virtual host with a wildcard ServerAlias.
In the server configuration, the virutal host with the wildcard ServerAlias directive would have to appear after the virutal hosts for the two subdomains.
It's an advanced setup that DreamHost doesn't offer. They would have to set it up manually and I doubt they would do so for inexperienced customers or those that can't demonstrate a real need for this functionality.
Edit: See this article in the Wiki - http://wiki.dreamhost.com/Wildcard_DNS