Add a charge controller to all in one inverter?

I’m looking at a system for a friend who has his set up using an all in one inverter, charge controller, utility input and inverter all together. He saw that my oversized system works even in the clouds using all independent charge controllers and he wants more solar available for the clouds and rain we have here and I was wondering if he could just connect another set of panels with a separate charge controller connected to the batteries? No communication other than the settings for charging and voltages matching. Right now the only connection between the batteries and the inverter is the battery cables. It seems to me that it should work just like mine does, all off of the battery voltage.
Also thinking about this made me wonder. If it would work, could I connect multiple all in one inverters each connected to their own solar panels, that don’t communicate, to a common battery bank, each powering an independent electrical system? I was thinking with the larger battery bank under normal conditions I could operate everything, then if there’s a storm or a major power outage I could cut off the extras and just have all the solar power go to my critical load system. the dog

If it’s all based on battery voltage and there is no (CANbus) communication to the batteries then there’s no reason why it won’t work. What Inverters are you using, the Voltronic units?

How do you measure battery SOC? Do you have a battery monitor (shunt) on the battery? Or do you just use voltage as an indication of battery level?

I have Growatt inverters for all of my systems except the one I will use as a slave only, that’s a Must, it isn’t working right now I need to find someone who can fix it here in PH. The main one is a 12kw split phase for our critical loads. I’m using New Green Tech charge controllers. They have been working fine like this. All I do is start them on batteries first, then turn on the breakers to the panels and they all seem to work together fine, their screens always show a voltage reading within a 1/2 volt of each other. Sometimes one will charge more than another sometimes the other way around. I finally got my Lithium rack batteries connected, I use the voltage as an indicator of charge, each of my batteries has a screen that shows the charge and the volts and they will shut off over/under volt. They all are within 1/10th of a volt of each other every time I look and the charge never seems to get above 25 amps each and the discharge never seems to get above 15 amps each. I have 100 amp breakers on each and a 250 amp main breaker to the inverter.
My friend is using a Must inverter and he was thinking of adding a controller they sell here, Aspor or something like that. He is thinking of doing it the same way I have mine set up. It made me think, why not just add inverters with charge controllers to the same battery bank, to me, all they will see is the voltage.