Treat Laravel multi auth users as a unique user class

I have been struggling with this problem for couple of days and I've searched everywhere but couldn't find a logical solution.

I need multiple types of users in my project(admin, customer) because I need completely different backend logic for each type. So I decided to use multi-auth method in laravel(which AFAIK is the best solution for these cases). So I have multiple user classes for each type(and multiple tables in DB) including Admin and User classes. AdminAuth and UserAuth classes manage the Login and Register logic and routes are handled using middlewares.

Up till now there is no problem. The problem is that I need to use a single user class in another classes. For example consider the messaging logic(and there are many many similar use cases):

a Message class Model should have:

protected $fillable = [
    'from_id', 'to_id', 'content', 'state'
];

 public function sender(){
     return $this->belongsTo(**User::class**);
 }

 public function receiver(){
     return $this->belongsTo(**User::class**);
 }
 ...

In the above model, I need to specify the User::class for senders and receivers, which can be either admins or users. So how can I tell Eloquent to use both models. Is it even possible? If not, what is the solution here?

I thought of using a higher level class named Person, for example, to hold the Admin or User object instances, but this way ORM can't manage to retrieve or store users from/in the appropriate tables automatically.

Any Suggestion is greatly appreciated.