When showing a friend how to tie some parachutes dry flies, he was having a very difficult time splaying the tail fibers and keeping them splayed. A very simple fix is to use a loop of thread (or any other small diameter material) at the bend of the hook. Pull it from bend through the tail fibers to split them. To split the tail in half, run the thread evenly between the fibers. Then tie down the excess thread and you have a perfectly split tail.
Please note there are 3 tail fibers in this example. When you go to split 3 fibers, you pull the thread such that your loop straggles the center fiber.