One way to control the number of columns the ePub reader uses is with book's Adobe XPGT stylesheet, but I don't know much about how it works. See my posts in
this thread for some examples.
Edit: If you wanted to make it so that it always used one column in portrait mode and two columns in landscape mode regardless of font size, then I think the way to do it would be to set ade:min-page-width to just over half the width of the screen in pixels, say 721px for the Forma.