input: Write a function to find the greatest two numbers from a list of integers. The function should return the two greatest numbers in descending order.
  assert get_greatest_2([1, 5, 3, 7, 4, 7, 8, 9, 2, 10, 6, 8])==(10, 9)
  assert get_greatest_2([5, 3, 7, 1, 4])==(7, 5)
  assert get_greatest_2([2, 2, 2, 2])==(2, 2)
  assert get_greatest_2([1])==(1, float('-inf'))
  assert get_greatest_2([])==(float('-inf'), float('-inf'))
output: def get_greatest_2(lst):
      if not lst:
          return (float('-inf'), float('-inf'))
      max1 = max2 = float('-inf')
      for num in lst:
          if num > max1:
              max2 = max1
              max1 = num
          elif num > max2:
              max2 = num
      return (max1, max2)