OK, I got the second propane tank in, and connected it to the original
with copper tubing and compression fittings. No leaks! Except at the
pressure regulator. Each tank has a regulator, they are connected
downstream of the regulator on the original tank (basically following the
directions of R. Jones (?)).

Now, when I open the valve on the original tank, LP gas escapes from the a
vent on the regulator on the second tank. This regulator is different from
the original one, and I'm not familiar with it - it has a small circular
screen facing down, with a cover riveted to the stone guard. It has a
white threaded cap covering a clear plastic cap which holds a spring down
against a pressure relief valve, I presume.

So, my best guess is its time for a new regulator. Anyone else care to
speculate? (And save me $30? Please?)

cheers,
Jeff in Missoula, where its far too cold to be doing this sort of thing
without a garage...