Wow. At first I thought that each was playing on three strings and was impressed with the arrangement of the music to make that work so well. Then I realized that they're mostly playing in a much more complicated way. She is picking what he is fingering and he is picking what she is fingering. It's not only a well written arrangement, but their timing has to be perfect. (They'd kick butt at Guitar Hero. :-) )
The following link is to something that looks similar but has people picking and strumming the same strings they are fingering. Fun .. and a good arrangement, but not as amazing. http://www.youtube.com/watch?v=xBqOjCt940g