Interference is a central feature of wireless communication.  The
standard engineering approach to dealing with interference is to
orthogonalize the transmissions. By designing the interfering
systems jointly, better rates of communication can be achieved as
shown by results on interference channels. Further significant
gains can be realized if, in addition, the radios cooperate
dynamically.  We study this form of dynamic cooperation using a
two-source, two-destination model where the source/destination
nodes can cooperate with each other by virtue of being able to
both transmit and receive. We characterize the sum capacity which
reveals a reciprocity between the cases where the sources
cooperate and the destinations cooperate. The main contributions
are novel strategies and new upper bounds.