a couple

The word couple poses some problems when we deal with subject verb agreement.  First of all, even with the phrase a couple, we have to be careful.  In American English, we expect a couple to be singular, which means we add an -s to the present tense verb that follows it.

Example: John and Mary are a couple.  This couple is traveling with us to Jacksonville on Saturday.

So far, we are doing well with singular subject verb agreement.  However, consider the following sentences.

They/It disagree/s about where to eat, so we went to separate restaurants, me with the husband, my wife with the wife. 

Obviously, in this example, it doesn't make sense because the couple we refer to in the previous sentence now becomes two.  This shift leaves us with a choice between remaining consistent or adjusting as we write. 

Consistent:  John and Mary are a couple.  This couple are traveling with us to Jacksonville on Saturday.  They disagree about where to eat, so we went to separate restaurants, me with the husband, my wife with the wife. 

Adjusting:  John and Mary are a couple.  This couple is traveling with us to Jacksonville on Saturday.  They disagree about where to eat, so we went to separate restaurants, me with the husband, my wife with the wife.

I like the second choice, adjusting, because I understand the couple to be a single unit when they are traveling, but they become two, the meaning of couple, when they disagree about the restaurant.