Pieter v D

Right ... Except there is no reason a battery module gets overcharged because the other one would have stopped charging. Batteries BMS limits voltage & current for each batteries independently.
If one battery stops charging, the other one will only accept voltage & current within limits of its BMS. Same for discharging : within the limits of each battery BMS.
If load > battery capacity : use candle. Communication between inverter & BMS does not change the problem ...
The only way to solve that issue aka good design is oversizing. If ever batteries may develop such issue ... Which is unlikely when the only job of a BMS is to not allow over or under charging or discharging.

Thank you @Coulomb and @Shadders.
Can confirm that this works on my Kodak 7.2 kW by following Shadders PDF and using the communication tool link in the same doc. 

Thank you @Coulomb and @Shadders.
Can confirm that this works on my Kodak 7.2 kW by following Shadders PDF and using the communication tool link in the same doc. 

Hey, neat! I have linked your file at the top of the calibration post. Thanks for doing that; it was sorely in need of a clean up.

These instructions need cleaning up, but all the information is there:
https://forums.aeva.asn.au/viewtopic.php?p=71631#p71631
Being MAX models, you will need the ATE1 command. Use the communications tool.

I shared the SOP I wrote up using the information linked above
 

There is a huge thread on it.
Summary: Most Axpert Max firmwares (except for a few "experimental" ones), and all VM III and VM IV firmwares, have a bug where the PV voltage can get stuck at a low value, usually close to the bottom of the MPPT range (90 V or 60 V), particularly in low light conditions (early morning, late afternoon, cloud, and rain). In the morning, the voltage will eventually shoot up from say 92 V to say 276 V, tripling the PV power. All that time from dawn through to this voltage rise, you've been missing out on up to 2/3 of available PV power. In some cases, on cloudy days, you can miss out on 75% of available PV power. But on a sunny day, the loss might be negligible, depending on many factors.
Georg594 pioneered a patched firmware that largely fixed this issue, with the downside that you end up with a fixed minimum PV voltage. So someone with 4S panels needed a different patched firmware than someone with 8S or 10S panels. I extended the patched firmware to other models. It looks like Voltronic have fixed the problem in a way that works for all panel configurations. But for whatever reason, they don't port the good code to "mainstream" firmwares. I'm able to recognise the presumed good firmwares, and list them here.

They are not cross compatible in that sense, no.
Yes, it should still be possible to parallel them. It's best if the firmware versions are fairly close to each other, in the sense that they were compiled from largely the same source code, but unfortunately Voltronic don't help with this.
Just updating to the latest you can get is probably fine. But if  you are concerned, ask your supplier(s) for the latest firmware update files.

The communications cable pin-out should be the same as the Greenrich LiFePO4 to the inverter which are Pin 3: RS485B and Pin 5: RS485A on the inverter side and Pin 7: RS485A and Pin 8: RS485B on the battery side.
I found the info here and it works for me with the Greenrich battery to the Kodak 7.2 Kw inverter, the Greenrich 3.686 KWh looks exactly the same as the Kodak 3.6 battery so I think they are manufactured by the same factory and looks exactly the same inside.
 

BMS issues pertain to the display firmware. I believe that this one (despite the lack of leading 1) is the latest and should work for you. I'm assuming that your model has the removable display. 
 

Fyi this just means the bms uses the pylontech protocol to communicate with the inverter, there are quite a few batteries out there that do.

Considering the OG 7. 2 as mentioned above. 

Then maybe sell the OG5.48  and buy a single OG7.2  , or if absolutely must a OG10 . The OG7.2 is very robust , I have one myself . I  refer to it as the the LandRover Defender of inverters , without the oil leaks though.
I am not particularly in favor of parrelling of machines . The mean time between failure is at best half of a single unit  due to the double amount of  electronic semiconductor devices. Remember these units does not introduce dual redundancy due to the fact that they share  the loads. Then on top of that there is the additional functionality of synchronisation and arbitration , functions that can fail and cause damge to both machines.

No not the OG5.48, only the OG5. 48a . The first one is equivalent to Axpert VM3, the second one to Axpert MKS3. 

Here you can see the Kodak King in Action. It can charge you battery and supply your loads at the same time. If solar isn't enough it will also use Grid to supplement load and charge battery.