Adding a different type of the same book to the same book location
Sometimes I have two copies of the same book, in different formats. Let's say that at one time I have a txt version of a book, I load it into calibre. Sometime later, I find the lrf version of the same book. I want to add it to calibre in the *same location* so that when I see the book I have TXT and LRF as book types.
However, I find that I can't, because when I upload the second book into calibre, it warns me of duplicates. If I say no to the warning, nothing gets loaded into the library. if I say yes, I have two library entries for the same book. I want the differing format book to go into the existing library entry. Is this possible?
edit: won't the ISBN need to be the same? what if there is no ISBN?