ATmega328 - two clock outputs
I'm not deep enough into the atmega family, but you can probably do the 1MHz with a PWM unit. These are programmable, so you can adjust them to varying system clocks if necessary.

The 8 MHz will be harder, but if you can generate integer factors of 20 MHz:


generate 4 MHz square wave
convert square wave (which has a lot of odd order harmonics) to sine wave with very simple RC or LC low pass filter
rectify sine wave » signal with frequency content at twice the sine's frequency (amongst others)
filter rectified sine wave to only contain the desired 8MHz sine wave
use comparator or Schmitt trigger to convert sine wave to square wave, if latter is desired