i would preferably like to use as little solenoids as possible this year and would like to know if you can run multiple pistons of solenoids
Sure, there’s no rule against it, just a rule against using multiple solenoid valves to drive one cylinder.
Yes but you will decease the air flow to the two cylinders in half. That may not matter for smaller cylinders but it will make larger ones slow.
Another issue to consider is that the cylinders will extend and retract in a staggard pattern due to only one cylinder moving at a time due to imbalance if the air pressure in the system which will move the cylinder with the least amount of resistance first. If they are hooked to a rigid enough structure that affect will be minimal, but in a flexible(less rigid) structure it could get bound up and not move after the first cylinder extends.