Use an islolator and don't worry about it.
https://www.amazon.com/Ampper-Batter...-11&th=1&psc=1
Now it doesn't matter equal or not, and if your battery goes dead just manually select the back up battery.
Boats have done this since the beginning of electric on boats.
Dollar wise, this is easiest and cheapest and fail proof.
(finding a place to mount that second battery might really suck)
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