If you wish to send commands to more than one GoPro at a time, simply add all of the associated SyncBac PRO units to the same group. You can learn more about creating groups here.
Once you have added all units, select the group (A-F) from the Group Menu on the left hand side. Any commands (on/ff or start/stop record) will now be sent to all units. You can also use the Group Settings menu (accessed via the icon in the bottom right corner) to amend the settings of all GoPro cameras in the group.