can model "belong_to" two other models and have a nested relationship?

Is it possible for a model to belong_to, two models and have a nested relationship?

i.e of what i want

class trainer
has_many :appointments
end

class appointment
belong_to :trainer, :customer
end

class customer
has_many :appointments
end

at the moment i have only the customer and appointment models which are nested e.g of what i have:

create method looks like this:

  def create
    @appointment = @customer.appointments.build(params[:appointment])

    respond_to do |format|
      if @appointment.save
        format.html { redirect_to([@customer, @appointment], :notice => 'Appointment was successfully created.') }
        format.xml  { render :xml => @appointment, :status => :created, :location => @appointment }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @appointment.errors, :status => :unprocessable_entity }
      end
    end
  end

in routes i have:

  map.resources :patients, :has_many => [ :appointments, :visits ]

is it possible to have 2 nested relationships for 1 model? what would i have to change my create method to, if appointment also belonged to trainer as well as customer?

thanks