I've always turned on both tanks and watched the indicator and have never ran out of the second tank.
It's hard for me to wrap my mind around the idea that it's better to open one tank and wait till my fridge, furnace, and hot water heater have quit, and then go turn the other tank on. What if your gone when the single tank runs out.
I recently had this discussion at a rally with several that preferred the single tank theory. I just cant see the logic, it just seems like backward thinking. It's so much easier to just check the changeover valve occasionaly.
Anyways, that's what makes life great, You can always do what floats your own boat ! RVing is addictive and there is no known cure! LOL !!!