Watch the Adding a Second Voice online guitar lesson by Brad Carlton from Guitar Lab: Boogie-Woogie
This lesson is oriented around the 6th string rooted E7 form barre chord. Add upper voice of 3, 4, 5 is played in top of the basic 5, 6, b7 move of the boogie-woogie. This layout allows for some "let ring" sounds. I have labeled this lesson "adding a second voice to the boogie-woogie" to show how to build on the basic two voice form. I am disregarding the root in the bass as you may play these patterns with or without that lowest voice.