Create a Python program that takes two string as input and produces a new string as output. The output string is the concatenation of the two inputs with all the vowels removed.
string1 = 'Hello World!'
string2 = 'This is a test.'
def remove_vowels(string1, string2):
  res = ''
  vowels = 'aeiouAEIOU'

  for char in string1 + string2:
    if char not in vowels:
      res += char
  
  return res

print(remove_vowels(string1, string2))