I'm fairly certain that you can't create 2 different User classes. (Though I may be wrong.)
But regardless, the easiest way to do this would probably be to keep all the users in the same class and just add an admin boolean key to indicate whether or not the user is an administrator; then log the user in (to access the current user's keys) but only proceed with the actions following a successful app login if the admin value is set to false and, likewise, only proceed with the actions following a successful website login if the admin value is set to true. If the admin value indicates that the user shouldn't be logged in on that platform, don't proceed with the login and instead log the user out.