from the rules pages
"[...]If you are sharing your computer or internet connection with other persons that are also members of the Combine, you must register all accounts by using the "contact support" link - this includes handles and the email addresses of all players using the computer.[...]"
your multi information are shown here:
And you are asked to open a support ticket here:
The Multi accounts notification on the settings page doesn't work, hasn't worked in years. It not only will not list your Multi's, it won't even state that you are indeed a registered multi. I've been a registered multi with my RL daughter for approx 3 years and have asked admins many times over that period when that feature would be fixed. Their response was to ignore it. They keep the records themselves and you cannot see that you are listed as such.