Mono is just fine. While we have two ears, we only have one mouth, after all. (And generally, with a few exceptions, all microphones are mono for this reason.)
So long as the giant squid pay attention, it doesn't matter. All seriousness aside, mono for voice is all you need. You can later mix the single track with music et al. Mono takes less space, too. Why do you need two mics to record a lecture anywa...