I have limited experience with multiple inverter setup. I had one user of my software https://gitlab.com/Magnum_Energy/distribution
that had a multi-inverter and multi PT-100 setup but due to circumstances outside the scope of this software he was unable to implement and validate the software. However he did have contact with Magnum Energy and I was able to indirectly ask some questions about reading multi device data. The answer I got, again indirectly, was that the ARTR and inverters are supposed to work this all out between themselves and display the net result. Also I was told the ARTR sent similar packets to the RTR. But this is third hand information.
I assume you have set up the ARTR as shown in the user's manual, page 4, Figure 2-1, System Overview. I recommend you look at data from communications port 5 or port 6 or perhaps the remote port on the slave inverter.
You talk of "data stream". what do you mean by this? Are you using my Magnum reader soft ware or something else?
The latest documentation I have from Magnum is dated May 2013 but does not include the ARTR. The document name that I have is "Magnum network protocol 7-13.pdf". I was given to me from an outside source so the official file name may be different.. I was asked not to redistribute it. If anyone has a newer version I would be very grateful for a copy.
You should verify this with Magnum energy but I think the Mag-Web interface does NOT support multiple inverters.
Re the Morningstar controllers. I am not using an undocumented way of extracting data. I used the Morningstar published modbus documentation. The code I use is published in my software project, referenced above, in the examples folder. It is posted as an example as I am not able to give users significant support. The code only extracts data points that interested me at the time I built the software. The controller has many, many data points and I ignore most of them. I can suggest solutions to puzzles and I try to correct any bugs I detect. I run my system with a slightly different version that I do not publish.
Using two controllers introduces an interesting problem of identifying data points. The software, as written, does not consider the possibility of more than one controller. You would have to decide what points are common, what points are unique and discretely labelled and which points to ignore. That is mostly a design decision and some code changes, not a technical problem.