Some clarification with proper terminology will help us help you. When you say two cylinders, I think you might be referring to the rules that limit you to two air reservoirs which, although they are cylindrical shaped, are not specifically called air cylinders. The thing you are calling a “bi-directional piston” is properly called a double acting cylinder. There is no limit to the number of air cylinders you may use on your robot. The only limitation is two reservoirs at 100 PSI pressure. Typically the reservoirs are connected in series, or connected together using a tee fitting. Adding the second reservoir will not increase your pressure, it simply gives you more volume of air for more cylinder actuations. There is no practical way to connect one reservoir to one side of a double acting cylinder, and a second reservoir to the other side of a double acting cylinder. The two inputs of the double acting cylinder both connect to the same solenoid valve, which is connected to the air supply provided by one or two reservoirs. If this doesn’t make sense, perhaps you could provide a sketch of your pneumatic system showing the reservoirs, the solenoid valves, and the air cylinders, and we might be of additional help.