But how could a 1MHz signal on XTAL1 give rise to a signal of about 12Hz on CLKOUT ?
Note: I tried removing the void loop() and just putting on() at the end of setup() and that resulted in no clock signal being output on CKOUT. There was a clock signal being output on the source Arduino, however.
That should have been exactly the right thing to do. Try doing it again, but also reduce REQUIRED_FREQUENCY in case 1MHz is a little high, given that you have the resonator still connected and a 1.5K resistor in series with the clock signal. As the Arduino you are using to generate the clock is also 3.3V, I think you can also safely try it without the 1.5K series resistor. What you are looking for is a signal on CLKOUT at a reasonable frequency.