Redirection to subpage URL ( help?)

Hi All! I need to redirect a customer's second domain to a subpage on their wordpress site. This works fine but they don't want the main URL to show as the subpage URL, they want it to show only the new domain. Example: Site is at duplex.com New url is triplex.com and want to point to duplex.com/triplex but should show triplex.com Hoping that makes sense. Domains are already pointed to the server. Is this at all do-able in cpanel or other? Help? Thanks! Am told this can't be done, just reaching out if there are other solutions I haven't thought of... Thank you!!