you _could_ do that, but then you are limiting the functionality of the search, and later on, if you ever want to search for foo OR bar, you wont be able to, you will be looking into searching only "foo bar"
IMHO, it would be better to add a note to the search form letting the user know that a space looks for either word, a comma looks for both words, and using quotes looks for the string. by forcing quotes around the search term you remove the possibility of ever using the either word search, or the both word search functionality.
