It seems that Contacts in Mountain Lion has dropped the Spouse field. I don't recall ever using a Spouse field, but I assume that it was there since you're using it. I mention this becsuse I was going to look at what might be done to solve your question and was surprised not to find the spouse field on my system. No Custom field either.
So, I think there might be some options. One would be to edit your Address Cards to move the spouse information into the First Name field so it would read Christopher and Brenda, before attempting the Mail Merge. I can understand why you might not like that. I used to do family cards that way all the time until it got to the point that so many of our friends had his and her email addresses that they needed separate cards so they could be addressed independently when creating emails.
Another approach would be to edit the Spouse names to add a distinctive prefix to the name, one that could be used in a Find and Replace operation. For instance, if "Brenda" became "_Brenda" in the Spouse field, you could use Find and Replace in the Merged document to substitute " and " for "_".
Lastly, you could drag your entire Group to Numbers and in Numbers create a merge source column that combined the couples' names in a way that could be used as one reference in the Mail Merge from Numbers.
Hope this gives you some options to consider.
Thanks, Jerrold. Your post gave me a very workable solution. In my letter template I have included:
Dear <first name> and <spouse name>,
Once I merge data from Address book, letters addressed to couples end up looking like:
Dear Christopher and Brenda,
Letters that have only one addressee end up looking like this:
Dear Christopher and ,
From there, per your suggestion, all I have to do is perform of find/replace where " and ," is replaced with "," and then all of my single addressee letters end up:
Which is exactly what I'm looking for. One more step than should be necessary, but in the overall scheme of things, it actually works very well.
Thanks a million!