Allow multiple fields to be controlled by the same master field

I'm not sure if this is a bug or a feature request.
I have the following situation:
1) I have a content type called "Activity"
2) Each Activity must be part of a Program, and has both an activity Type and an Objective
3) The Types and the Objectives are different depending on the Program

I set up three vocabularies: Program, Type, Objective. Type and Objective both have a reference to Program
In the Activity content type, I then add term reference fields for Program, Type, and Objective, with Type and Objective both being dependent on Program.
However, when I create an Activity node, choosing a Program only limits the options for one of Type or Objective, but not both - which is the desired effect.
I put this as a bug because the system does not prevent you from having two reference fields delimited by the same master field, but if this is the correct behaviour then it should be an enhancement request.
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