I have slit FFCs under a microscope: I needed some 50-pin, 0.5mm pitch cables for prototyping but had only 40-pin cables available, so splitting and taping them back together to the rescue (while waiting for the proper cables)! Tidy your workspace, use a sharp model knife under a good stereo microscope, and then carefully pull the cable, keeping the knife steady against the table. Do it slowly and control the pull direction carefully to keep the slit going in the middle of the tracks, avoiding sudden, large corrections.
No problem with the result, but this is a very tedious task, so only suitable for very small runs (or expensive products, where manual, skilled labor is required anyway). Expect 5 minutes of skilled, careful work per cable, and expect a poor yield.