input: Write a function to find the two greatest numbers in a list.
  assert get_greatest_2([1, 5, 3, 7, 4, 7, 8, 9, 2, 10, 6, 8]) == (10, 9)
  assert get_greatest_2([4, 1, 7, 7, 3, 2, 8, 8]) == (8, 8)
  assert get_greatest_2([1, 2, 3, 4, 5, 6]) == (6, 5)
output: def get_greatest_2(lst):
      if len(lst) < 2:
          return None
      first, second = float('-inf'), float('-inf')
      for number in lst:
          if number > first:
              second = first
              first = number
          elif number > second:
              second = number
      return first, second