In a perfect world, the modules would move at the same time and 50% of the force would go to each module. However, if you don't have even weight distribution front-to-back, you may see one module move before the other. In this case, you'd have >50% of the force going into each module.
Having not seen the entire CAD of your robot, you may not have room for this. You could use one piston per module, and have one end of each piston on a pivot on your frame (Show in yellow). This way the pistons would have a much larger lever arm to work with.
