Put your friends caching name in "Compare with", select the option "Both found". You probably want to add the filter "Show Disabled/Archived". When you do the search you get tabs under the map, one with your name, one with your friends name, one says "none" and one says "both". Click the on that says both.
This gives you a search for all the caches that you both have logged but they don' have to be on the same date to show up. How much work it is to go through them manually depends on if you've cached a lot on your own in the same area so to speak.
