You get one column on epub books if you tap on the largest 3 font sizes. (In fact, I never saw the two column pages before because I read on the third largest font size.)
Are you finding 2-page displays on pdf's? If so, go to settings, then applications, then tap on reader. Make sure "enable 2-page PDF display mode in landscape" is unchecked.
On magazines, it's one page in portrait mode, and two pages in landscape mode, with zoom capabilities, as far as I know.
Had the 2 column un-checked, but today - first time - had a 2 column display.. WOW, but you answer was the correct one,