"Built-in oscillator isn't designed to drive longer signal traces or additional capacitve loads, but only the crystal and its associated burden caps sitting extremely close to the oscillator and ground pins of micro."

a quote (I do not remember which datasheet/user guide): " TAL2 may be used to drive one CMOS load"

Since this is a quote from "a" datasheet, that, of course does not oblige Atmel to make XTAL2 CMOS level for any new device.

I, personally would never use that technique (I do not make toys) but in any case where more than one processor is involved I use a separate clock chip. If you must use the uC clock as a 'general' clock source make absolutely sure that the trace to the clock buffer is ultrashort and include the buffers input capacitance in the capacitor calculation for the XTAL2 cap. Again, I would never use that thechnique and oscillators are not THAT expensive ( a quick look at digikey show an 11.0592 oscillator selling for $1.12).

Running multiple processors from each their crystal can easily spell disaster.

Erik