Electronic – Looking for 10GHz range clock generator

The normal method for using PLL to multiply frequency is analogous to the normal method of using an op-amp to multiply the voltage of a high-impedance signal: the non-inverting input is fed the input signal directly; the inverting input is fed a scaled-down version of the output. The op amp will vary its output voltage as necessary to make the two inputs equal.

Likewise, when a PLL is used for frequency multiplication, the "non-inverting" input will be fed from the reference signal, and the "inverting" input will be fed from a divided-down version of the oscillator's output. The oscillator will attempt to vary its output frequency and phase as needed to make the frequency and phase of its two inputs be equal. Suppose, for example, the "inverting" input is driven by a divide-by-four circuit. Since the oscillator will have to output a frequency four times that of the reference input to make the frequency of the control inputs equal, that's what it will do.

This can be done but it requires extremely careful analysis to make it work reliably.

You should generally tap your signal off the XTAL-OUT (a.k.a X2) pin as this is the output of the on chip oscillator circuit and drives a higher signal amplitude and the driver will have lower node impedance. This is particularly important if the XTAL-IN (a.k.a X1) and XTAL-OUT pins are connected to an actual crystal. Added capacitance, particularly at X1 pin, can change the operating frequency of the oscillator circuit or prevent it from starting up reliably.

Tapping either the X1 or X2 pins requires consideration that these are analogue signals and do not normally traverse between full digital logic levels. The tap circuit load needs to be high impedance and needs to be able to translate the low level analog signals to the type of signal needed for the load at the next circuit in line.

If you instead are driving the XTAL-IN pin with some type of external oscillator device it may already be providing a full logic swing signal. In that case the external device may very well be quite capable of driving to the XTAL-IN pins of multiple MCUs with no problems at all.