120 RPM = 2 revs per second. With only 1 magnet that gives an update frequency of twice per second. Why do you seek an update of 16 times per second, or is 120 the maximum and you want to measure very much lower. 1 magnet at 120 RPM will give 2000+ hours of reed switch life so why not go for the simple solution. If balance is a problem you can always fit a "dummy" magnet opposite the real one.
I once used a reed switch on my father's car as a tacho pick-up, operated off the ignition coil so doing up to 2000RPM and it lasted the life of the car