input: Write a function to find the greatest 2 numbers from a given list of integers. The function should return a tuple containing the two largest numbers in descending order. If the list is empty, the function should return None.
  
  def get_greatest_2(items):
      if not items:
          return None
      max1 = max2 = float('-inf')
      for item in items:
          if item > max1:
              max1, max2 = item, max1
          elif item > max2:
              max2 = item
      return (max1, max2)
  
  assert get_greatest_2([1, 5, 3, 7, 4, 7, 8, 9, 2, 10, 6, 8]) == (10, 9)
  assert get_greatest_2([1, 2, 3, 4, 5]) == (5, 4)
  assert get_greatest_2([42, 17, 54, 29, 61]) == (61, 54)
  assert get_greatest_2([]) == None
  assert get_greatest_2([23]) == (23, float('-inf'))
  assert get_greatest_2([15, 15, 15, 15]) == (15, 15)
output: def get_greatest_2(items):
      if not items:
          return None
      max1 = max2 = float('-inf')
      for item in items:
          if item > max1:
              max1, max2 = item, max1
          elif item > max2:
              max2 = item
      return (max1, max2)