/usr/share/doc/muttprint/sample-muttprintrc-en has:

# Here you can turn on or off the paper save mode
# (printing 2 pages on 1 page)
# Set it to 'optional' if you only want papersave mode
# if more than one page is printed.
PAPERSAVE="off"

Both "off" and "on" work as described, but if I set

PAPERSAVE="optional"

it has no effect, i.e. it behaves like the default "off"
(tested on a mail with 5 pages).