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.