This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
from Dirk W. Howard
My wife and I have two All-American canners. One can do a double stack of
pints and a single stack of quarts, and the other can do a triple stack of
pints and a double stack of quarts. I like the large capacity. It means
that in a 75 minute processing time I can do anywhere from 9 to 18 pints in
the "smaller" canner and up to 27 pints in the "large" canner. Total would
be 45 pints if running both canners. As opposed to 375 minutes (3 hours 15
minutes) to process 45 pints in a single 9 pint canner. OK, this isn't fair
since I did gauge two canners on one. Let's say that your goal was to
process 36 pints of green beans. In a single stack canner that is four
different batches. Just the processing time alone is 5 hours. This doesn't
count the vent time and the cool down. A canner that can have a double stack
of pints cuts the processing time down to 2-1/2 hours. This can be worth the
extra price of the canner and the trouble (minimal) to work with.