I found a workaround! I simply converted my book into a PDF and am reading it in Side by Side. There is only one book because I chose to buy the original text with notes, but there are so many notes that it's highly impractical to switch back and forth between text and notes. And MarginNotes doesn't display them in a satisfying manner, maybe because there are so many. Anyway, I know it may sound crazy to convert an ePub to a PDF, but this way I may actually be able to read the book
So I have the book opened twice in Side by Side, one side with the text and one with the notes. I even opened a third window to access a Middle French dictionary that I found online, but I'm not sure my screen is big enough to do this comfortably.