When adding a new domain name to an eZPublish website, the site.ini file must be modify to assign the domain to an existing siteaccess.
Simply add a new entry into the HostMatchMapItems array:
vi settings/override/site.ini.append.php
HostMatchMapItems[]=<domain_one><siteaccess_fr>
HostMatchMapItems[]=<domain_two><siteaccess_en>