In the 3D program I've got, upon selecting a vertex you can use sliders to set the influence of each bone. It appears you've got it right, because it's acting the same way my 3D program does - holding down either mouse button is like using the sliders. If the points on the right are weighted to 1, then they're controlled less by the bone on the right and more by the bone on the left and visa versa.
Edit 3: Sorry about the stupidly written post. Everything in your .cap behaves exactly the same as in my 3D program.
Setting everything to one will cause the entire mesh to be moved by that one bone, exactly like you have it.
Setting everything to 0.5 looks exactly like if you set the weights on each sprite to 0.5. The mesh rotates and shrinks.
It looks awesome. The only option I might ask for is if there was a way to keep the thickness uniform as well, like in Drasa's 'spine 3.cap' here: http://www.scirra.com/phpBB3/viewtopic.php?f=16&t=1482&st=0&sk=t&sd=a&hilit=spine&start=20