I'm working on a weird body-type: a monster that's basically two torsos joined together, with a third pair of arms at the "hip" area where they meet. So sort of a six-armed centipede character with a neck & head at each end. I think I attached a screenshot, if I did that right.

His bound spine is controlled by a separate driver chain, which extends from the bottom neck base to the top neck base. That driver chain has a spline IK, running from the bottom neck base upward.

The spline curve is smooth-bound to three joints: the spine bottom, mid, and top. Those joints are then parented to control curves.

My sticking point is rotating the middle or bottom of the chain. The last point in the chain (the base of the top neck) stays put, because that chest control curve isn't moving, so the last bone stretches while the rest of the chain follows the curve.

My only limitation is that the bound joints all have to be connected, since this rig is for a game. Is there a better setup I should be using for an anatomy like this? Or is there something I can change about the rig to prevent that last bone from stretching when I bend the spline IK?