System 1 allows a single-user to finish both sides of a door at once by themselves. The "Spray Transfer Cart" is not only for a single-user though. This cart is one of our favorites because a user can walk the door to a user's ergonomic height which is 2" below the elbow. A user will sand a door at one height and then walk the door down to spray the door. Another benefit that most people do not realize is you can spray the door vertically or at any angle they want. It is better to spray at an angle to save on a user's back and shoulders. "NEW ITEM" We came out with a 360 door lock that locks the door into any position! This door lock works on both "Spray Transfer Cart" and "Adjustable Cart."