cr0sh:
Also, if you want to use the clock source (system clock - which I believe is any of the clock sources, based on my reading of the datasheet) to drive another device, you get that from the CLKO pin (PB0, pin 14 on the 28 pin PDIP).
I just tried that, and with a 16 MHz Uno I got a 16 MHz output on PB0. So you could have a single crystal (or resonator to save the capacitors) on processor A, configure it to output its clock, and take the external clock on processor B. That would also keep them exactly in step.