To make columns in an ePub, use the following CSS property on any block element, such as "body" or "div":
oeb-column-number: [integer] | auto;
from the ePub OPS specification:
Specifies a number of columns in which to render content; may be applied to all block level elements. Reading Systems are free to support integer values other than 1, or may map them to 1. Reading Systems may support column balancing. A value of auto allows the Reading System to decide on the optimal number of columns in which to render content, considering available width, font sizes, or any other metrics it considers relevant for readability.
Anyway, I tested "2" and "auto" on my prs-600 and in ADE. Both worked, although the prs-600 "decided" just to use 1 column. I had to make the font size very small in ADE to make AUTO display as 2 columns. However, hard-coding 2 into the CSS made 2 columns display on the sony reader.
Pretty cool! =)