On guitar, Powell fretted with her right hand and Simpson plucked the strings with his left. On bass, Simpson fretted with his right and Powell plucked with her left. Simpson and Powell eventually got back to their own instruments during the solo. Powell explained the situation succinctly.
My left hand is doing the bass strings, my right hand is doing the guitar fretboard. Dave’s right hand is doing the guitar strings and his left hand is doing the bass fretboard. This was just for fun.
via The Awesomer