2 boxes at one_half do not fill page width

I want two boxes to span the width of my page with a small margin between the two boxes.
My page width is 960px and I have 2 boxes with widths of one_half and one_half last. Content and title of both are aligned left. Each box displays at a width of 300px and there is a 50px space between the two boxes. This means I have a remaining 300ish pixels displayed as an empty space to the right of my boxes.

How can I adjust the width of these boxes so they are truly half of my 960px page?

http://wordpress.org/extend/plugins/boxer/