External crystal needed for ATmega328P for multi-master I2C communication?
Multiple IIC devices do not need synchronized clocks.  IIC is itself a synchronous protocol.  During any one message, the master controls the SCL (clock) line.  As long as the master lets the clock dwell long enough at each level for the slaves to have time to properly interpret SDA, there will be no problem.

So no, you don't need crystal-accuracy clocks for IIC communication.