If I'm team roping, just a single rein. Its just easier of me to have a smooth run with less in my hands. Ranch roping I use Romels, mecate, whatever the horse I'm riding is in.
LOL, I only ever use split when I'm roping, not really by choice though. I ride all my horses in split reins and I only ever do ranch/pasture roping. I don't often have the time to stop and change out my reins when I find a cow that needs roped.