I've never tried the double rein thing, but I think you are right that it would be better at teaching cues from the ground. But it would be daunting all those lines:) I don't know that I have that much co-ordination:)
If I ever started a young one, I think i would try to learn that:)
Great pictures, BTW:)
Becky
The double reins are nice for training, because you can go from lunging, to ground driving, back to lunging without stopping. And they learn to deal with all the gear, and the lines, and the tangles you inevitably end up in, and learn to keep their head about it.