You can't hook the crystal up to two AVR processors at the same time. What you CAN do is connect the crystal to one AVR and have it output a buffered 16 MHz clock signal that can be used by the other AVR.
You will need custom fuse settings, both for making the first AVR output the system clock on the CLKO (PB0) pin and for making the second AVR use an external system clock on the XTAL1 (PB6) pin.
Another option is to use a 16 MHz oscillator module and feed that to both AVRs as an external clock input on their XTAL1 pins.