There's no way to do this with CSS. It's the bane of designers everywhere.
The solution would be to use PHP to split the content into two equal(ish) chunks and put them in the separate columns.
Keep in mind, however, that having columns on a web page does not have the same advantages as it does in print. In print, it's nice to be able to see more text without having to turn the page (i.e. you just move your head back up to the top). However, on a web page it's quite tedius to have to scroll back up every time you get to the bottom of the first column. So it's in your best interest to only use columns for text when the amount of text is very small.