This method uses a pair of iFrame elements. It doesn't work very well given the constraint of PBWiki, but I might be able to fix that with one of the pay versions that allow for custom style sheets.
Still, it's pretty ugly . . .
The best way to handle this type of commentary is to use the "discussion" feature of MediaWiki. While both texts aren't on the same page, I think it's the most straightforward and usable way to accomplish this type of activity.
Standalone pages on a server that link into a wiki page would work well too - but then the texts and the discussion would be in different places.
Perhaps a wiki isn't the best solution here. A blog post for each chapter would allow students to read the text and then leave comments about the text afterward. While this doesn't allow for the same interactivity and students aren't able to edit the comments of others, it might work well.