You can't mirror a subdirectory. A redirect will change the browser's URL from "personal" to "commercial." That won't harm your Google ranking, but then it is no longer "personal."
One option is to set up a Fully Hosted personal domain and tell it to use /home/USER/commercial.com/personal for the web directory, instead of /home/USER/personal.com
Just be sure that you're not using full URLs for links on your site(s); just a relative link and that will preserve the desired domain in the URL.