A characterization of the capacity region of a two-way channel is given for the communication situation, in which both senders send independent messages simultaneously to both receivers and all senders and receivers are at different terminals.
"The Capacity Region of a Channel with Two Senders and Two Receivers." Ann. Probab. 2 (5) 805 - 814, October, 1974. https://doi.org/10.1214/aop/1176996549