I would not worry about him being on two reins at the same time. If he is listening to the inside one telling him to stay soft to the inside, he will actuallhy be "off" that rein and ON the outside, which just kind of "breathes" with him, no more. I always found that tricky; keeping my outside rein steady, but not so rigid that I stopped breathing with him.
Oh, and bending in the ribcage isn't really bending. It's really the inside leg stepping under the barrel , which causes the barrel to roll outward. Same dif.
Post a video sometime. Would be fun to watch!