Which of these would you say?
The married couple who the police caught is on drugs.
The married couple whom the police caught are on drugs.
I would say
First, I would use whom because it is the object of the phrase whom the police caught. The police are doing the catching; the couple is being caught.
Second, I would use are because each individual in the couple is on drugs. Being on drugs is not a trait of the couple as a collection but instead is a trait of the people making up the couple.
I would say:
Most native speakers of English do not use whom except, perhaps, in formal writing.
Couple can be treated as singular or plural, depending on whether the speaker wants to consider them as two separate people or one group of people. Drug taking habits tend to be a personal matter, so I treat them as two separate people.