This solution isn't working for me.
I created and moved over the ABRelatedNamesConfig.plist file to the appropriate location in Contacts.app's Package.
I'm running 10.8.2 as well.
Using the John Smith/Jane Doe example, both entries have the exact same address (with no typos), the full names of each spouse in the related name/spouse fields exactly corresponding and spelled the way that they are on their own cards.
Do the country fields in the address need to be filled? I tried "US", "us", "USA", and "United States" anyways, and it didn't make a difference.
What makes me think it is not working is the preview window in the print dialogue panel doesn't show any "John Smith and Jane Doe" but just "John Smith" or "Jane Doe" - just the name of the contact on the card.
Was the xml/plist file supposed to add an option in a drop-down menu somewhere or something? It looks to me that the xml/plist file is just a case/switch scenario checking if the country flags true for either "us", "uk", or "fr" in its unaltered state.
Perhaps when you first created/moved your file you did it on a previous version of OS X, and subsequent upgrades allowed it to still work in Mountain Lion? I'm on a MacBook Pro from 2010 and went from Snow Leopard to Lion to Mountain Lion.
My "spouse" is hovering and insisting that I just do our holiday cards by hand or one by one in Pages, but I am VERY stubborn and want to get this to work.
Thanks in advance for any help.