string1 = 'This is a test trees are green roses are red'
string2 = 'This iS a TEST trees 12.48.1952 anthony gonzalez'
str_lst = string1.split()
for s, t in zip(string1.split(), string2.split()):
    if s.lower() == t.lower():
        continue
else:
    str_lst.append(t)
string = ' '.join(s.lower() for s in str_lst)
How to find union of two strings and maintain the order