Believe me, one is enough. I agree, the Rock just don't lend itself to a sidecar. The Bonnie does.
I'm trying to look at the mounts. The back one looks to be a Motorvation Engineering mount but the front one, I'm not sure about. In normal engineering practice, when mounting a chair, because of the torsional stresses involved, you need a four point mount. The back mount is right, one low and one high, but the front isn't. Besides, the front mount has been bent and that's a big no no. All the mounts should be straight with no bends or stress points. It's triangulation. If you look at my bike in my album pictures, you'll see how the mounts are and that's what you need.
It also seems as though the chair is leaning in. The chair should sit a vertical plane with the bike leaning out slightly. This outfit is seemingly opposite and that might be why it's for sale....It might have handling woes that the owner can't correct.
I'm sure he has some bucks in it. I know I do with my little Bonnie and the chair.
I believe Tony has seen it in person, haven't you?