When they roll the reveal video leading up to the vote outcome, they show before and after shots of each duplex, with the before shots showing the spaces as they were before any work was done, including showing belongings that were left behind. That would not have been possible because the sponsors of the home and garden show were not even aware of the flip competition until both sides were well into the process. At best the before shots would have shown spaces that had already been through "demo day".
When Jules does her first walk through of her home, Lance is already renovating his unit on the other side and comes crashing through the common wall. The shot shows that it is a 2X4 wooden stud wall with drywall on either side. This is not possible because fire codes require that there be a non-flammable wall between separate housing units sharing a common wall. This wall is always made of concrete, masonry or cinder blocks, so Lance could not have crashed through.