You can't own two copies of the book at the same time: that's the whole point of the concept. Yes, you can make copies of the physical book, but even today that's not very important - the book industry is doing well. What's more, you can imagine a bookshop that offers a two-way street: you like having an e-book but then you find it good enough to buy it in physical form. In that case, you add a few dollars more to get the physical version.