Since a recorder is a single network device, it is monitored by a single TetherBox and cannot generally be split across multiple units. This poses the question: "What if I need multiple TetherBoxes to record a single recorder?"
The workaround for this is activating each unit in a separate location (How can I activate a new TetherBox?) on the same timeline, e.g. "South Office 1", "South Office 2":
Note: If you already activated the TetherBoxes in the same location, you will need to factory reset those units.
Once the units are activated in individual locations, each TetherBox will pick up the recorder as a single camera. You can then convert this camera to a DVR, selecting the number of required channels.
You may need to adjust the paths in the advanced tab to record the desired cameras: