If want to control all the domains, before they resolve, then that could be done by creating some symbolic links. Let’s say you have domain12.com and domain13.com, which you want to to access. You’d run the below commands as the user through ssh:
rm -f public_html
mkdir –mode=755 public_html
ln -s ../domains/domain12.com/public_html domain12.com
ln -s ../domains/domain13.com/public_html domain13.com
Alternatively, if you just wish to switch which domain is being used for /~username, you can do so by logging into the User Panel -> Domain Administration -> select the active domain to use and click “Set as Default”. This will setup the symbolic link for the selected domain for /~username.