There is a lot of hype around the splitting of threads to form a dubbing loop. In my opinion, if you are using fine enough thread, you can simply create a loop in the thread, and you, or anyone else looking at your flies, will never be able to tell the difference.
To create a loop, simply use your fingers to hold a loop of thread away from the shank, and return the bobbin lead thread, to the shank, wind it around the base of the loop so formed, and continue winding. You could also introduce a thread loop using a separate thread of finer diameter than that which you are using to tie.
I use 14/0 thread most of the time. On delicate nymphs I use one of the “spider threads” for the dubbing loop.
One advantage of making a loop instead of splitting the thread, is that the winding up of your dubbing in the loop, will definitely not wind up the thread you tie with thereafter, meaning that it will continue to lay flat. There is a technique for ensuring that the split thread doesn’t remain wound up (and rope-like) after wrapping your dubbing, but I don’t think it is failsafe.