I often simulate assemblies with hydraulic cylinders in them.  I can simulate the cylinder, once fluid is closed off in it, as a rigid link.

But often there are pairs of cylinders, spaced apart, which remain in fluid communication with each other after the control valve is closed.  So, the total fluid in them remains constant but one could send fluid into the other if the assembly they are in bends or twists.  (Our frames are necessarily open C-sections, which we know twist quite a bit - sample pic attached)

In this case I want to set up a constraint like a coupled pair of rigids, where total length (A+B) remains constant.  Is there a way to set this up?