I use my left tank as my primary tank and the right tank is the secondary. If the primary is low and I know I will run the heater during the night I'll switch over to the secondary before going to bed and back to the primary in the morning.
Once the primary is empty I switch to secondary until I get the primary filled. I'll reinstall the full tank to the right and move the used secondary to the left. Now he is the primary and my secondary is full. Start the process over again.
This has been my system from day one (20+ years) and I'm sticking to it.