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John Ryan on the Arduino forums says:
Here's 2 ATMega168's sharing one 16mHz resonator, one reset switch, two 10k resistors, one 220ohm resistor, and two LED pilots. I used a mini USB adapter and uploaded the LED blink test to both chips, and as Paul [Badger] suspected, they are perfectly synchronized - I've tested numerous resets, and the start up blink sequence and blink demo on both, are identical.
I don't need synchronized chips, but I happen to have a use for 2 168's connected via i2C - not sure if anyone else does but it adds a lot of pins and a second serial buffer for an almost next to nothing outlay. This can be put together using 1 RBBB and a second chip with the boot loader installed, so that's quite inexpensive, so long as you've got got a mini USB adapter laying around.
Synchronized ATMega168s sharing compenents on the Arduino forums, via Hack a Day.
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Totally freaking sweet.
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The only thing you must worry about when using a single crystal, VCO, or otherwise clock generator, is that the lines coming out of your generator must be close to equal (tolerances vary directly compared to clock frequency), else they could end up in frequency-sync, but out of phase, which could be as bad.
A visual confirmation of sync (two LEDs blinking at the same time and not drifting apart) will tell you if you are running at the same frequency, but not necessary if you are 'exactly' in phase (exactly being the same rising clock edge).
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