Via reg:exp, Oroboro by Björn Hartmann and Jennifer Carlile is:

OROBORO is a novel collaborative controller which focuses on musical performance as social experience by exploring synchronized actions of two musicians operating a single instrument. Each performer uses two paddle mechanisms – one for hand orientation sensing and one for servo-motor actuated feedback. We introduce a haptic mirror in which the movement of one performer’s sensed hand is used to induce movement of the partner’s actuated hand and vice versa.

This is an amazing example of how tangible interaction techniques coudl support innovative joint activities. Applied to other context (i.e. ... video games) this would be nice.