Trying to make a 2-column list which scrolls on demand, and doesn't break the layout when the scrollbars appear is rather fiddly in CSS.
This solution uses a UL, where the LI elements are set to float left, and the width of each is set to 50%. Note that I've used the CSS3 property box-sizing (or -webkit-box-sizing) to make the 50% widths include the border and padding. Without this the total width of two 50% elements with a border adds up to more than 100% and the layout breaks.
Note that the height of each element must be set explicity.