To run them in parallel like that, they do indeed need to match.
The isolator is to prevent what's called a "circulating current". If you start and run the vehicle every day, neither battery will run down far enough to create a problem. What we see over and over is that when the weather changes, or when the vehicle is parked for a couple or more days, one battery will self-discharge a little faster than the others. Now, the other battery will try to 'charge' the one that ran down first. This is the "circulating" current. They'll do this until both of them are flat.
What you can't predict is how long you'll need to park before this happens. Might be three or more days. If you live in a climate that has four seasons, the first night the temperature falls to freezing, this will exaggerate the small differences in self-discharge between one battery and the other. Folks around here learned to unhook the extra battery when they'll be parked longer than a day or two.
Isolators are not magic. They will create a voltage drop, and you'll see less power when an isolator is in line. They are not fail-proof, and will break down if overloaded.
Best solution I know is the battery switch used in boats. Selects "A" , "B", "Both" or "off". Just gotta remember to select only one battery before you leave for a week's vacation.
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