I design a board with two STM32, Can I provide a clock from the "master" MCU to the "slave"MCU. I connect the master MCO1 pin to the slave OSC_IN pin ? Can the master reset the slave with NRST pin?

[TDK]

Yes and yes. That clock would be HSE bypass mode. Doesnt have to be MCO, it could also just be a timer.

[Tesla DeLorean]

Yes, this is how many of the NUCLEO/DISCO boards work, and connect with the ST-LINK STM32F103 device.

You could also share a common clock between units, which might actually be preferable.

You would need to drive NRST independently, the GPIO would need to be in open-drain mode as the pin is bidirectional, the STM32 can reset itself (POR, WDG, etc). Driving the NRST pin high with a push-pull driver will likely result in undesirable operation.