I think I understand your problem. Try adjusting CSS line 577. By default, the two column layout is set for a right column of 50%, which is obviously too big for a sidebar and makes it look like there’s an empty column to the right. Go to Theme Options, then Layout, and try adding the following custom CSS in the window where it says “enter your custom CSS” to make the sidebar smaller:
.twocol {
width:30%
}
Adjust the percentage as you wish, but I found that 30% looks good for a sidebar.
Hope that helps!