The Doctor seems clear to me - when you perform a discard threat marker action, you discard two of the same colour.
This will be the key text. You need to perform the specific discard a threat marker action in order to enjoy the Doctor's ability.
However, one of my fellow players had the Plasterworker and the Doctor. The Plasterworker allows you to discard a threat of a colour when you play a house of that colour. My fellow player said since he was doing an action (building a house) which discarded a threat, he should be allowed to discard two.
"Doing an action" is not enough...you need to be doing the specific discard a threat action in order to benefit from the Doctor.