Here's my benchmark (in case I made a mistake):

  import time

  def combine(upper,lower): return (2**32)*upper + lower
  def combine2(upper,lower): return (upper << 32) | lower

  def test_combine():
  	start = time.clock()
	for i in range(10000000):
		combine(i,i)
	end = time.clock()
	output = 'The time taken for combine() is ' + str(end - start)
	print output

  def test_combine2():
	start = time.clock()
	for i in range(10000000):
		combine2(i,i)
	end = time.clock()
	output = 'The time taken for combine() is ' + str(end - start)
	print output