Adding a second matrix, just repeat the hardware you have with exactly the same pins appart from its own chip select line. Use that to address the second display.
Addressing a single pixel - look at the code you already have and the data sheet for the 7219 that will tell you what you need to set a single pixel. Basically you will have an area of memory defining the pattern you are displaying, you have to change bits in that memory and then write it out to the chip.
There is no subistitute for knowing what you are doing, cut and paste coding will only take you as far as people have been before.