That’s not really BMS related. It is a new feature that allows a BMV or a SmartShunt (which is a more compact BMV) to measure things other than a battery. It can measure dc loads, alternators, fuel cells, and so on and so forth.
One of the new features is that you can measure your “DC system”. You would put this shunt between your battery and your DC loads, and then instead of getting an estimate on the GX overview screen (which is the difference between the AC power and the DC power… ie whatever gets lost is assumed to be used by some DC load), you will get a real reading.
With a real DC system reading, you can adjust the solar chargers to compensate for that (in cases where the BMS requests a current limit). You can even let the Multi compensate for the DC loads, in systems with AC-coupled PV and a charge current limit.
Because this intended to be used on boats, where on a Catamaran you could have two sides… it allows you to have more than one DC-system shunt, and they are added together.
But it does not add battery shunts together.
You can however use the idea to make multiple BMSes work together, and present it to VenusOS as one unified “battery” service. Which is what the commercial batteries do, except they do it on the CAN-bus. Not in software.