that's a very interesting idea, and it seems like ebooks would be great particularly if you are using one with a dictionary. i think for the formatting, 2 columns might be a bit difficult on a 6" screen, because each column would be very narrow and you would have to use a pretty small font to fit more than a few words on each line of the column, but if you want to play with it you could try creating a word document with a page the size of the screen, then insert a table with two columns, and put one language in each column. then you could make your word document into a pdf. this would work better on a device with a larger screen, of course, and those are coming, so depending on what device you have it could be a good solution.
alternately, you could try one paragraph in language1 and one in language2, using different fonts (serif / sans serif) for each language, but i don't know how comfortable that would be to read and it might be a lot of work.
there might be other options ; perhaps someone else will have some ideas.