It's interesting how easily the mind can be fooled with some clever editing. This was performed back in the 1930's by a guy named Johnny Eck, although I think that Criss Angel does it with contortionists and editing rather than someone with sacral agenesis.
In the pull apart video, at 1:58 you can see the bulges of the girls' legs wrapped up under her shirt. I also don't think that the "audience" is random either-the people pulling the bodies apart don't react as I'd expect them to. If you were pulling on someone's arms, would you continue pulling when you noticed the bottom half separate from the top? Most people would immediately stop pulling before the bodies were several feet apart.