I agree with Steve on this. We dont have a clear idea on whether Google considers Hyphens and Underscores as two different parameters or the same. So, to be on a safer side, why not make a single character change in the URL, which would make a bigger difference and solve the problem completely. Moreover you have mentioned that both the contents are different, which is a good thing.
Having duplicate content on two subfolders on the same domain doesn't seem to me like a problem. Moreover, if you want to experiment, you must try the same link text, one separated with hyphens and other with underscores with either duplicate or same content and check out what Google says. Moreover, I dont think there is going to be a problem in SERP too, as Google does not just consider link text for ranking a particular page for a particular keyword.