There are two ways to hook up multiple tanks.. The first is the PROPER way far as I am concerned
Tank 1--Line to---Change over valve with check valves----Line to----Tank 2
And then of course you need REGULATOR
and then following that . . . YOUR STUFF
With this system the change over valve has an Arrow on a control, may be a paddle or a knob. the arrow points to the IN-USE tank. both tanks are valve open When the pressure in the IN-USE tank drops enough the gauge goes RED and the regulator draws off the other tank Flip the knob/paddle, remove the empty (Close the valve first) and re-fill and re-connect.
When the 2nd tank runs empty (you have already opened the first tank's valve of course) flip/turn and .. Well you can figure it out
Option 2, (What they put on my SCAMP
Tank---line--T---Line---Tank
The "T" fed the regulator.
No automatic anything.