"The couple will have died."
Translation:La pareja habrá muerto.
So, after "the couple"/"La pareja" we use the singular (el/ella/usted) tense of the verb and not the they (ellos/ellas/ustedes) tense. Fooled me there, I mean, there are two persons in a couple :)
Got me, too. Then I figured that there is only one couple, so it is singular...
No, one couple. And like many collective nouns in Spanish, "pareja" is treated as a singular noun requiring a singular verb ending. If it were "los dos" ( the two) a plural verb would be required.