it's a tough one that all image library applications I've tried struggle with. one of them offers a "closest match" option that while not perfect, does a not bad job of it.
I'm anxiously awaiting some neat new image catalogue toys to try out in the current 11.1 beta phase that may or may not tackle this particular FAQ.
In the meantime, there a couple of awkward work-arounds that might be of use to you...
I forget the members name now, but I remember reading how someone had set up a custom field they used to catalogue the number of people an image contained, so, selecting karen and ken in the people column, then "2" in this custom column would return the files you were after. Initially, this could require a considerable time investment if you have several thousand files, but after that, should be easy enough to maintain.
entering this in the search box:
[=isequal([people],ken;karen,1)]=1
will also return the files you desire.... BUT.... maybe not all of them..
it would appear that MC merely appends new entries in the semicolon delimited list, therefore, you may also need a seperate search of...
[=isequal([people],karen;ken,1)]=1
to show the rest. If you wanted a more permanent search on these two dudes, create two new smartlists, one for each of the two searches above.
then create a 3rd smartlist. Use the wizard to add the previous 2 smartlists to the search rules of this 3rd list and use it to get your list of images that show just ken and karen
-marko.