That's a crystal. Maybe not the best.
What I wrote:
With the internal oscillator calibrated you can be within 1% and with a crystal I think you can get closer but I dunno just how much -- and still the suckers won't stay in synch.
From ATMEL docs I see you can calibrate the internal oscillator which ships more like within 10%. I don't see a way to calibrate the external clock beyond changing the crystal.