The whole premise of multiple keyword searching (as it relates to a blog - in my opinion anyhow-) is to narrow the scope of the search to articles that include all of the key words entered in the search string. What you want to do sounds like maybe actually widening the search engine capabilities by including the ability to use boolean search operators. (ie.. AND, OR etc...)
Here is what came back on my plugin search;
//wordpress.org/extend/plugins/search.php?q=boolean+search
and a forum search ;
//wordpress.org/search/boolean+search?forums=1
//wordpress.org/search/advanced+search?forums=1
-at a glance, this looks pretty interesting
//wordpress.org/extend/plugins/enhanced-search-form/
Here is an interesting blog article posted in march of this year by someone with the same intentions.
That is a great question. I'm sorry I don't really know the answer.