Duo style is a very popular right hand technique. Buddy Wachter talks extensively about in his Homespun Tapes tenor banjo offering.
As I understand it, the idea of due style is this: Play a downstroke of a chord. Then do a series of rapid up and down strokes on the highest string of your original chord. Let's say you play an open C chord. Strum the C with a down stroke. Then play that C note (third fret; first string) with an upstroke, then a downstroke then an upstroke. (My example uses three single notes, but that's definitely not the only way to go.) If one play the pattern cleanly and evenly, he will produce a sound that resembles one player doing chords and another playing tremolo. (hence the name)
My understanding is that the duo style was first developed by Eddie Peabody. Yes, Buddy Wachter shows you how to do it on the Homespun tapes. I had the Plectrum Banjo one. He taught how to hit the full chord on the downbeats as well as the offbeats. Buddy Wachter - Highly recommended!
Duo Style is an older classical mandolin and banjo style (five string) that has been adapted to tenor banjo and other instruments. Crosspicking (at least modern bluegrass inspired crosspicking a' la' Tony Rice and Jesse McReynolds - your idea and mine might be different) doesn't sound like you are playing accompanying chords at the same time you are playing the melody. Duo Style really sounds like two or more instruments playing at the same time.
I'm sure that Harry Reser could play Duo Style so we may be talking about two different crosspicking definitions. Duo Style is the way that the old classical mandolin books (like the Bickford Mandolin Method of 1920) describe this technique and it is not the same as the crosspicking methods that the bluegrass players use. It has been around for a long time as the Duo Style well before tenor banjo players picked it up. (Or the tenor banjo even existed.)
Duo style and cross picking are both potentially fine techniques, but are not at all the same. For great examples of crosspicking, check out guitar flatpickers Dan Crary and Pat Flynn (along with the fine examples given by Mikeys). The most straight forward example, explanation, and demonstration of duo style I know of can be found on Buddy Wachter Homespun Tapes instructional DVD.