Multiple art dirs per script

I've used the "Creating An Art Pack" instructions to create art packs but I
would like for one art pack to be able to display multiple directories of
jpegs.  I know there is no automatic way to traverse nested directories, but
can I include multiple explicitly defined directories via the .roku script?
Perhaps something like this:

DESCRIPTOR_FILE="./dir1.artpack"
$ARTPACKER "$DESCRIPTOR_FILE" &
DESCRIPTOR_FILE="./dir2.artpack"
$ARTPACKER "$DESCRIPTOR_FILE" &
DESCRIPTOR_FILE="./dir3.artpack"
$ARTPACKER "$DESCRIPTOR_FILE" &
fi