Gavin has a tutorial on harmonizing two strings.

http://www.guitartricks.com/tutorial.php?input=240


A quick and easy way to harmonize in fifth and in octaves is to use powerchords. A powerchord is created by using a root note and a fifth. You'll often see people adding an octave of the root note.



G5 powerchord
--------
--------
--------
---5---- G
---5---- D
---3---- G


Using the powerchord structure has a reference point, you can easily find the fifth and the octaves. So if you want to harmonize in fifth, use powerchord in your riff and you'll find all the fifth and octaves you can use.


Simple exemple

Riff - riff using powerchord - riff using octaves

----------- ----------- ----------
----------- ---------4- --------4-
----------- ---4-5-7-4- --4-5-7---
---------2- ---4-5-7-2- ----------
--2-3-5---- ---2-3-5--- ----------


You can do the same exercise for fifth.

Edit : Thx for the tip Christopher!