Following up on the instances of two-headed cats, here’s two heading people:
Abigail and Brittany
http://video.google.com/googleplayer.swf?docId=8951987143067325335&hl=en
Rock Band fronted By Conjoined Twins
http://video.google.com/googleplayer.swf?docId=421053855597493100&hl=en