One of my horses has a very long thick mane. I would never want to cut it. For show I do a running braid. It takes a little practice but now I can do a nice neat, tight running braid in only 3 minutes. I do it everyday before I ride him, just so I can see his neck and not get my reins tangled in his mane. I have never tried a double running braid on him, but I did last week on my other horse who has a fairly long mane and it was a failure. Guess I'll have to practice that more.
The trick to a nice running braid is to keep it tight while braiding it. Taking smaller sections of hair helps, but the main thing is to give each section a tug up tight to the roots after you cross it over the previous section. Hold it there as you work with the next section. If your horse moves around while you are trying to braid, it will usually result in a lumpy or sagging braid, so learn to do it fast. I keep a small hair band wrapped around my 2 middle fingers of my left hand as I braid so I'm ready with it when I finish.
Note: In preparation for braiding, I carefully brush the mane with a large pin brush starting at the ends and gradually working up to the roots. I don't want to pull out ANY hair so I'm careful and I use as much Cowboy Magic Detangler as needed. Once the mane is smooth, I can separate out each section with just my fingers as I am braiding.