input: Write a python function to count and print the total frequency of characters in two given strings.
  assert char_frequency('apple', 'pplea') == {'a': 2, 'p': 4, 'l': 2, 'e': 2}
  assert char_frequency('hello', 'world') == {'h': 1, 'e': 1, 'l': 3, 'o': 2, 'w': 1, 'r': 1, 'd': 1}
  assert char_frequency('test', 'testing') == {'t': 4, 'e': 2, 's': 2, 'i': 1, 'n': 1, 'g': 1}
output: def char_frequency(str1, str2):
      from collections import Counter
      counter1 = Counter(str1)
      counter2 = Counter(str2)
      combined_counter = counter1 + counter2
      return dict(combined_counter)