I get asked if bookmarks can be engraved on both sides, and the answer is yes! I do use real veg-tanned leather and hand dye and seal each item. The front side of the leather is shiny and will patina and the back side is rougher, but it can still be engraved.
These photos illustrate the difference between the front and back texture. I personally like to use the back for more personalized messages or dates and the front for the design or quote, but there are plenty of options!