2 piston 1 valve?

Can we use 1 valve for 2 pistons? We didn’t notice any rule prohibiting this…

Yes. In many cases this is a good way to control the pneumatics.

For example, if you have pneumatic shift transmissions, then controlling the pistons with one solenoid assures you that both will shift at the same time.

This is not prohibited while the opposite is prohibited.
Bruce

Just remember you are going to get a reduced flow rate if you do this.