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.