The deal with running on 1+2, ALL, BOTH (whatever your switch says) is not when everything is good, it's when something isn't good. If you have an undetected electrical malfunction, you may kill or damage both batteries. Also depends on how your system is wired and how you maintain your batteries.
I have two identical batteries, but only one is in use at any given time. The one in use is primary, and the other is a fully charged 'reserve'. On my boat, the use of BOTH is an emergency procedure dictated by the primary being unable to crank the engine, and the outing is terminated once the engine is started. Something drew the primary down, and I'm gonna get home before the same thing happens to the reserve. It's what works for me ... :wink: