What those meters I mention do is not only tell the SOC% (I always ignore that and have a Whizbang Jr- there is no Sr) and voltage but also current amps in and out and amp hours remaining in real time. Real time means they track both charge AND discharge from the batteries. They're the bees knees to really know what's going in and out of your batteries. For example, they'll show I'm using -5 amps, turn on a light, now -7 amps so that light uses 2 amps to be on. If I was charging at 10 amps, and using 5 amps the meter would show +5 amps going to the battery. So Oldcat your 2 6v @ 205ah you'd program the meter for 102ah as total capacity. When the ah=0 you're at 50%.
I'd recommend the Expion360 meter over the Trimetric for 'works on any battery configuration meters' simply due to the simplicity of Expion360 over Trimetric. There's a thing in programming- never allow the programmers to make a UI as it will won't make sense to an end user. Trimetric didn't heed that wisdom- the install sheet is confusing, way too many wires and using the unit means a small learning curve. The Expion however is a piece of cake to understand how to hook up, the wires have 1 connector to the shunt that goes to the unit and 1 other wire hitting battery + for power, programing is simple and there's some other perks in the display- only problem is the size of the display unit is too small.
For Midnite controllers the Whizbang is simple to hook up and the controllers already have a screen built-in for the information. Unfortinatly the 30a Kid info pales to a Classic, plus the Classic keeps the Whizbang screen on all the time while the Kid don't.