In the case of positive trust, would it be right to credit multiple positive feedbacks to an individual (from multiple users) with a single trade? No. However, in some cases, a user may have some a major thread where they're doing good for the forum (i.e. rizzlarolla, TwitchySeal) that could warrant positive trust.
Also, if you've had multiple positive interactions with a user, then you yourself could potentially give multiple positive trust ratings.
Remember that there's actually very little written down about what you can and can't do: there's only really guidelines.