Arduino UNO with an extra Atmega328 chip in same project

As subject state, Arduino UNO with an extra Atmega328 chip in same project How would one go about it. Based on the following article MasterWriter I would assume that a single arduino uno with a simple Atmega328 chip would also be possible in a project? Thanks

The Uno is designed for development, smaller boards can be used in final installations. And yes, I2C can be used for inter-processor communication.