The weight is attached to a pulley. Each rope runs through the pulley and is attached to the top of the box. Let $F_t$ be the total tension in the ropes connecting the pulley to the box, $F$ be the total force you exert on the ropes, and $F_g$ be the force of gravity on the weight. No matter how many ropes you pull, $F=F_t$ and $F+F_t=F_g$, so $F=F_g/2$.

The weight is attached to a pulley. The two ends are of the same rope that runs through the pulley. Also, the rope is tied to two ropes that connect it to the top of the box on either side of the pulley. When you pull one end of the rope, the pulley lets you cut the weight in half, but when you pull on both ends, you do not use the pulley and lift the entire weight.

Below is my attempt at depicting this contraption.