In general, you will probably not get good results on most devices when trying to do a two column layout in epub. There is no way of correlating one paragraph to another, which means you will probably run into situations where the page breaks do not work well if at all. In addition, many epub reading systems do not work well with floating text (which is how you would create columns).
If you'd still like to try experimenting with it, I would suggest breaking the text of each book into paragraphs, interleaving them, and having the text of one book float left while the other floats right. Look out for paragraphs that run off the edge of the page and do not continue on the next page, though, as well as overlapping text at the top of pages.
A quick and dirty example of some code you could get started with:
<p class="english">This is the first paragraph</p>
<p class="italian">This is the first paragraph</p>
and the corresponding CSS:
As for the initial conversion, you might check out this question.