Capacitive sensing between two copper plates
There is really no way to do what you ask.  Each copper plate is a single electrode.  You therefore have a single capacitor.  Something disturbing the field or coupling to the plates can be detected as a change in capacitance of the capacitor, but there is no way to tell where on a plate.  You can only measure a single value.  How do you suppose you get the additional degrees of freedom to determine location?

You can make a capacitively senses touchpad, but that is done with a array of individual capacitive sensors.  There are a lot of different ways to go about measuring the capacitance, inducing the system with test signals, layout of the array, etc, etc.  However, all these methods require multiple sensor capacitors to measure more than just touch or not-touch.