Technique suggestion: Only use the rear brake when you're making U-turns or other very slow-speed manuevers... use constant braking and increase/decrease brake lever force to work against pedal pressure: makes all the difference in the world. Master it and you'll find you can trackstand your tandem at short stops if you haven't already mastered that.
As for the front brake studder, you might want to:
a. Adjust your headset, i.e., make sure there isn't any play
b. Clean your front rim brake track with some alcohol on a scrunge pad.