AVR external clock problem
as long as you have the fuses set to use the internal oscillator and you are not using the XTAL pins for any reason on those chips, you should be A-OK. You should not set those XTAL pins to outputs however, but rather inputs without internal pullups enabled (i.e. DDRx = 0; PORTx=0). On a separate note though, I'm not so sure you'll be OK with more than one crystal per chip. The crystals are 'passive' components, and the AVR actually has some circuitry in it for 'driving' the crystal to resonate...



EDIT
OK I just wanted to make sure you weren't trying to "share" crystals. As the others have stated, I can't imagine a (sane) reason why you would have to run your chips at the same clock rate. Be mindful however, that some of your register settings (for example timer prescales, baud rate settings, etc) have implicit coupling to your clock rate, and may need adjustment in otherwise identical firmware.