For a screen, I'd plant them about 4' apart. But, unless they are really large now, it's going to take a number of years to hide that shed. You may want to consider planting more than two...

In all the photos I have seen of this bamboo, it seems to grow as wide as it is tall. Are you sure 4 feet is not going to be too close? I am starting out with 5 gallon 5 feet tall plants so yeah it will be a while before it is huge. :)

It all depends on how much screening you want and how soon. You can certainly plant them further apart, especially if you're going to use only two bamboo. I'd say you should simply set the pots out in different locations and try to visualize what it will look like in 5+ years if they expand in the shape you're familiar with. I guess if you plant them pretty far apart and aren't satisfied in a few years you can always put another one in the center.