FINALLY! I get to offer YOU advice! Hahhahaha
I rode in split reins before switching to English. First, they do hang down quite a ways. If they are hitting the hock they may be too long, but they definitely go past the elbow.
If you are direct reining or two-hand reining, I suggest you take the left rein over the neck and off the right. Right rein over the neck and to the left. Hold the two reins in each hand just as you would a single loop. I liked to have about 12" of slack centered between my two hands.
If you are neck reining you can do the same thing, only now one hand centered over the withers OR let both ends hang off the same side of the horse, your preference. I would do this and then carry some of the hanging slack in my spare hand (to give it something to do and because I thought I looked cool and calm.)
I uploaded a photo of me in my new English saddle trail riding.... I have the reins slack and off to the same side. They are almost at the hock, but when we begin moving I know I gather more up so they are not that low.
I think it is in the review of my new saddle (wintec 200 AP). I would repost it, but my iPad has a hard time adding photos to existing threads.
Now, the WHY. An old cowboy once told me that having the single loop was not as safe as two reins. Eh. Not sure if I buy that anymore (I am no longer THAT green) but, having two reins is sort of handy.
You can jib a horse on one of the reins if he is acting stoopid on a trail.