Yes it is a multi you two are. Making a support ticket is the way to go, so good on you. Support tickets have been broken for months now such that emails are not sent. However so long as you made the support ticket the Asims will check that before any banning goes on.
In relation to this question, I regularly use the wifi (which is huge) at university to log onto the Combine. I don't fully understand the technological side of things here, but say another player connected using that wifi, would that get me banned?
"Civilized men sleep soundly because rough men stand ready to do violence on their behalf..."
Usually the asims can tell if you are on something public like that, the IPs are different. And if they aren't they will just watch the accounts, and if they arent interacting in a similar manner or acing like multis you should be fine. No need to sign up as a multi if you dont know if someone is on your network.