The Guidance Computer is responsible for determining the trajectory of a rocket so that it can reach its destination (hopefully in one piece).
A manned rocket requires exactly one guidance computer. The guidance computer requires either a planet ID chip or a space station ID chip to successfully travel to its destination.
If a planet ID chip is used then the destination can be selected by using the "Select Planet" button in the main menu of a rocket
If multiple guidance computers were built into one rocket, one would override the other, however it is impossible to know which one wins out before you arrive at your destination.