You can punch in any two players, and it will retrieve their completed achievements for both. You can hide mutually-earned or mutually-unearned achievements, or show only the achievements that are exclusive to player 1 or to player 2.
I should note that the achievement database I created includes guild achievements (at this time), but the Armory doesn't return completed guild achievements in player profiles, so those will always show as incomplete (for now). Also, I know a few of the icons are broken, due to spacing/underscore differences between the game data files and what the Armory API returns.
Let me know if you have any questions or comments, or design feedback or suggestions.
Hopefully someone else finds this useful!