You are talking about a multi-processor system with a master-slave architecture. Basically what you do is program a bundle of useful subroutines into the slave uC, and have the master uC command the slave to execute them over some communication bus (i2C, RS-232 etc). You decide on some protocol by which the master can tell the slace which subroutine to execute and have data exchange between the two.

Its a pretty useful thing to do, since you can divide the complexity of the task between the uCs. With a bus like I2C. you can expand the system to as many slaves, and even masters, as you need.

Kundi