I2C at 2.4 MHz with five microcontrollers over 2 meters
There is a highspeed specification for I2C, you can read about it here: http://www.i2c-bus.org/highspeed/ 

However, it needs special hardware and very few devices support it.

I2C fast mode supports up to 1MHz, but you have to be very careful about how much capacitance you have on the bus.  Two meters of wire is likely a non-starter.

So I would consider other interfaces.  SPI might be a good choice as EE developer mentioned.

All this assumes that you mean megahertz not millihertz :)