Manifold to connect two 20lb propane tanks

[KS Guy]

That makes sense.  I was thinking of my old Chevy pickup, which has 2 tanks, one on each side, but the valve splitting them went bad.  So we just put in a brass T instead of replacing the 150 dollar valve, and since the fuel pump is on the right side of the motor, it pulls from the right tank 3 times as fast as the left.  Of course that works off of suction, but I wondered if a similar thing would happen here.  

[ThomasPowers]

Also the tank with less propane will chill down faster as it has less to chill and that will slow down evaporation too.

The problem being that at sometime both will be empty.  I used to work in a Mud Logging trailer with propane heat and two tanks. We were supposed to use one till it was empty and then switch to the other and get the first refilled.  There was a massive snow storm and my boss asked me to go rescue two mudloggers who had opened both tanks at the same time and then ran completely out and were marooned as they had ridden out in their new Corvette and the ice on the roads was higher than its ground clearance.  My 1968 ford ex-phone company van with tiny motor but good clearance made it out with food, water and propane. IIRC I did have to use a come-a-long and pull it back on the road 3 times. (20' log chain with hooks and 40' of wirerope with loops on the end)  Never saw so many pheasants in my life; they were surrounding where the hay bales had been dropped by helicopter for the stranded cattle.