If you use the Nikkor 62mm polarizer on the 20mm, it will not vignette,
but since you want to use one on the 180mm also, it makes sense to get
only one (plus 62-72 step-up ring), given the high price of the Nikkor
polarizers (which have larger glass than the mounting ring size). The
HN-13 is a VERY deep, straight shade, and would seriously vignette on
the 20mm - which does not need a shade anyway. (It needs only the
occasional finger over the front of the lens to cover the sun when it
is just outside the frame.) BTW, if there is sky included in most of
your pictures, you will not use the polarizer much - the sky over the
angle-of-view of a 20mm will be VERY uneven in tone. Hope this helps.