You can learn about different intervals like 5ths, minor thirds, 9ths and that sort of thing. Then one guitarist can play the root note and the other guitarist can play whatever interval that creates the harmony you want. Theres no formula to harmony, you just have to know your theory. When you play a power chord, thats harmony. Bend some double stops, thats harmony. The guitar is one of the only instruments that can create harmony on its own.