No 2 batteries are exactly alike. Try comparing 10 fully charged batteries and note how the voltage is different slightly on each one. Especially if the batteries are not the same or the same age.
Wiring them so they are in parallel all the time - the weaker battery drains the power from the stronger one. Ending up with neither are fully charged.
Keeping a battery less than fully charged shortens the life of it considerably.
As they get older, the difference become greater, they drain each other more and more, both facing premature death - at the same time!.
Use an isolator solenoid to keep them separate when not in use. Wire it to the ignition switch so when the ignition switch is on, they connect together.
It separates the batteries but connects them together when running, allowing the alternator to charge them both.
A good place to get that solenoid is at a camper or motorhome supplier, like Camping World. Not expensive. They do it all the time.
Do an internet search - look up battery isolators to see how they are wired, and why.