Ok, I think I understand. You want to update a value at a specified address on a different MCU but using I2C as the communication method.
It is possible but I2C isn't the best protocol because it is primarily intended to be used for input and output rather than linking autonomous systems together. You need to emulate a slave device on the second MCU and then pass it more than one byte. The first byte(s) would carry the address you want to write to on the slave and the next byte(s) would carry the data to be written to it. So you pass the location and the content as a multi-byte I2C transfer.
Brian.