whatever rustle your jimmies...
which is the distance between microcontrollers?
I assume You can program both microcontrollers (it doesn't matter that they have different crystals)
you have to choose the slower one as the limit for the communication... both for a synchronous or asynchronous approach...
for an asynchronous approach look if both micros have an USART module, (then you can program each one with a common baudrate for each clock frequency (so the program will be a little different) if the distance is long, look for TTL to RS232 or TTL to RS485/422 converters (like the max232 or the max485)
for a synchronous approach look for SPI or I2C, I suggest you to use the SPI it's easier... (but the distances should be close, like <0.5m
The distance is around 30 cm now that I use breadboard. But the target is to put on the same PCB.
The problem is that for some reason the one microcontroller runs at 8MHz (and cannot change) and the other microcontroller runs at 16MHz (and cannot change). The second (the one with the 16MHz)cannot change frequency because it communicates with another machine at 38400 bps and that cannot change. The other (the one with 8 MHz) can only runs on lower frequencies!