At this time, Okta only supports prompt=login as a way to prompt the user to authenticate, but as you saw, this will only prompt the user for their password, not to completely relogin again, in part because we only support a single user having a session in a given Okta org in a single browser (unlike some providers that support a select-account option for the prompt parameter)
Far as I can tell, the best/only approach to ensure that users are challenged for a full re-auth would likely be to end any existing Okta sessions before trying to log in the second user