Multiple shared_ptr storing same pointer

Consider this program:

#include <memory>
#include <iostream>

class X
  : public std::enable_shared_from_this<X>
{
public:
  struct Cleanup1 { void operator()(X*) const; };
  struct Cleanup2 { void operator()(X*) const; };
  std::shared_ptr<X> lock1();
  std::shared_ptr<X> lock2();
};

std::shared_ptr<X> X::lock1()
{
  std::cout << "Resource 1 locked" << std::endl;
  return std::shared_ptr<X>(this, Cleanup1());
}

std::shared_ptr<X> X::lock2()
{
  std::cout << "Resource 2 locked" << std::endl;
  return std::shared_ptr<X>(this, Cleanup2());
}

void X::Cleanup1::operator()(X*) const
{
  std::cout << "Resource 1 unlocked" << std::endl;
}

void X::Cleanup2::operator()(X*) const
{
  std::cout << "Resource 2 unlocked" << std::endl;
}

int main()
{
  std::cout << std::boolalpha;

  X x;
  std::shared_ptr<X> p1 = x.lock1();
  {
    std::shared_ptr<X> p2 = x.lock2();
  }
}

I don't see anything in the C++11 Standard section 20.7.2 suggesting any of this is invalid. It's a bit unusual to have two shared_ptr objects store the same pointer &x but not share ownership, and to use "deleters" that do not end the lifetime of *get(), but nothing forbids it. (And if either of those are entirely unintended, it would be difficult to explain why some shared_ptr member functions accept a std::nullptr_t value.) And as expected, the program outputs:

Resource 1 locked
Resource 2 locked
Resource 2 unlocked
Resource 1 unlocked

But now if I add a bit to main():

int main()
{
  std::cout << std::boolalpha;

  X x;
  std::shared_ptr<X> p1 = x.lock1();
  bool test1( x.shared_from_this() );
  std::cout << "x.shared_from_this() not empty: " << test1 << std::endl;
  {
    std::shared_pt