Double notes!

I have been listening to Jason Becker and Rob Johnson for awhile. In their music I hear scary double note passages. I dont know of many other guitarists who use double notes. Is it possible to play complicated double notes real fast or could they be overdubbed?

My floor pedal has a pitch effect, where it copies the signal, shifts the pitch of the copy to a note of varying degrees above or below true, and mixes with the original note to sound like two different strings. That what you mean?