Data will never be sent over interconnect. There are two possibilities
1. Use NDMP three way backup. Data will be sent from controller B via LAN to controller A which writes them on tape.
2. Use drive sharing. In this case tape drive will be visible to both controllers and backup manager will "grant" permissions to use to to one or other. It may require extra licensing depending on backup application.
It is in principle possible to send NDMP stream from host to emulate 3-way backup. I knew some backup products that supported it. Whether BE can do it, is probably better asked on BE forums.