Yes, limelites has done it on at least one of his sites - you could look at the view source and the CSS to see how it is organized.
Essentially, you would make a new category group for the set of subcats you want in the second column, then give both groups widths just less than half of the total sidebox space. Probably the best way to put the second column beside the first is to give it position: absolute; and top: and left: properties to put it where you want it. Nothing else will be trying to occupy that space to the right of the first column, so everything should fit neatly.