The I2C to I2C communication is set by configuring the I2CSEL pin on the serializer and the deserializer. For example if you want to set up an I2C-I2C communcation between the MAX9291 and the MAX9278A, you have to set I2CSEL (pin 13 on MAX9291 and pin 38 on MAX9278A) high during the power up.