the only way I've found to get around this is using the search bar.
Click on the wizard arrow, choose more fields > people and in the dialogue select both John and Rachel and click OK.
You will now see the list darichman refers to.
Click on the wizard arrow again, select people again, and this time, "select all", then deselect John and Rachel, and finally, use the drop-down selector and change 'include' to 'exclude' and click OK. (de-select "only show items that match other search criteria" on each of the people dialogue)
Now, your list should only contain those photos that include both John and Rachel, or just John, or just Rachel.
Adjust the results by clicking each [people] search, choosing "edit with wizard" and ticking people as you see fit. If have a favourite list, save it as a smartlist. I asked a while back for this to be made a little more user friendly but it wasn't to be. I'm looking forward to the next beta cycle though, as I've heard on the grapevine that image handling could well be high on the agenda. Nothing is ever set in stone, but at least the mood is a positive one, which is always promising.
-marko.