How to mocking 'fork' method twice.

mocking_fork.rb

require 'rspec'

class Foo
  def my_fork
    fork do
      p 'forked once'
    end
  end
  def my_fork_double
    fork do
      p 'forked once'
      fork do
        p 'forked twice'
      end
    end
  end
end

describe Foo do
  subject{ Foo.new }
  describe '#my_fork' do
    it 'should call fork' do
#     Foo.should_receive(:fork) # incorrect
      subject.should_receive(:fork)
      subject.my_fork
    end
  end
  describe '#my_fork_double' do
    subject{ Foo.new }
    it 'should call fork twice' do
      pending
      subject.should_receive(:fork).twice # fail
      subject.my_fork_double
    end
  end
end