I simply open one and leave the other close. When i runs empty I switch to the other and fill the previous. I know of no other way to do manager LP with out totally running out.
As for the comments that a tank should last a long time is very relative. For us, in the summer months, when the furnace never runs we can get ~20-25 nights out of a single 30# bottle running the fridge and water heater on LP plus a little bit of cooking.
In the fall, with cool days and cold nights and the furnace running, I get about 6 day/nights out of a single 30# bottle.
At deer camp, when it is cold often with snow, I get a week out of two 30# bottles.
furnace use has a lot to with LP consumption.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)