Thanks, that is helpful information.
You are welcome.
I was looking up all those facts and you've summarized them nicely for me.
Atmel does a terrible job of providing a quick summary of processor information (in my opinion). I've always ended up back at the datasheets.
As for accuracy, one could probably drive them with a precise clock already available because of the other MPU.
Yes. Even the lowly ATtiny13 can be driven from an external clock signal (but not a crystal or resonator).
You could even use the crystal on the AVR and scale that clock down to drive the MPU clock input as I think the prescaler scales clock out on some AVRs.
You can but the AVR will also run scaled-down.