Gary,
My day job is running a company that builds bespoke hydraulic and pneumatic machinery. I can tell you that the biggest challenge in this industry is trying to synchronise multiple actuators (Hydraulic or pneumatic) connected you the same piece of equipment. The problem is that even tiny differences in friction, either in the cylinders or in the mechanism will cause one cylinder to move first. If the linkage is not stiff enough the differential movement one side to the other exacerbated the differential friction and the whole lot locks up. In hydraulic systems there are various types of flow dividing valves that try to equalise the flow to both cylinders and sometimes these will work but they are not 100% accurate and so small differential movements can still cause jamming. This type of flow control won’t work on pneumatic systems because of the compressibility of the fluid. The only way you are going to get this to work is if the mechanism can be made stiff enough to keep the cylinders synchronised. So in your case this means the cross bar has to be guided in some way that it can’t tilt with respect to the gear side plates. To prove what I’m saying you could disconnect the cylinders and see if you can twist the cross bar and get it to jam. I’m sure you will be able to. Difficult to see how you could stop it twisting, maybe large flanges that run inside the gear plates but then they would need to be much larger to provide the clearance for the flange. I think the best solution would be one much larger cylinder, centrally mounted to the cross bar. It would need to have a large enough rod to provide stiffness at the connection to the cross bar.
As I said, this type of problem is the most difficult fluid power issue that I have faced in over 40 years in this industry! Sorry I can’t be more positive!
Malcolm