The real question is why bother if all you're going for is dual-mono. All you've done is double the file size, unecessarily considering most playback devices will play a mono wav or mp3 into both L and R. There are a couple of options beyond dual-mono, like delaying one channel a tiny bit (I'm talking a couple of milliseconds here, if you delay too much, your brain will only hear the first signal in one ear and not the other channel despite the fact that audio is coming out of that side too). My experience with this has been mostly dissapointing -- now I usually just leave it mono.