input: Write a function to remove vowels from two concatenated strings.
  assert remove_vowels('hello', 'world') == 'hllwrld'
  assert remove_vowels('beautiful', 'day') == 'btfldy'
  assert remove_vowels('AEIOU', 'aeiou') == ''
output: def remove_vowels(s1, s2):
      vowels = 'aeiouAEIOU'
      combined = s1 + s2
      return ''.join(char for char in combined if char not in vowels)